tempted to renew the crank bearings, big end shells and all bushes and seals as a matter of course while I am in there, given the minimal amount of extra cost relative to the project as a whole.
Give in to that temptation. Given the history of the bike and the external state of the engine, imho there isn't any point taking it to bits and not
doing bottom end bearings; the only possible question mark is whether the big-ends need grinding undersize and new shells or just the latter. If it does need a regrind, needs to be done by a specialist who knows old Triumph cranks. Whether it needs a regrind or just for other spares, Rockerbox Motorcycles
isn't far from you.
frame & engine numbers match?
The engine number is TR6P BG46822, which doesn't match the frame number
Would it be logical to assume the engine (or at least the left side crankcase) has been swapped in from a police spec bike?
An educated guess says it's more likely to have been the complete engine, at least as far as the top of the cylinder block. To tell if at least the crankcases started life together, look at the lower mounting lugs - if both case 'halves' have the same code stamped on the lug, they started life together; if they don't, they didn't.
Then risking telling you something else you know already, the engine number date code is February ("B") '72 ("G"). That was originally OIF so do the Bonnie head and rocker-boxes also have the OIF mods. or are they pre-OIF, which should make attaching the head-steadies to the frame easier?
clearer view of the yokes
The Zener mounting lug is definitely 'factory'; three other things would confirm the lower yoke is T150: the steering bearings are balls - either 20 loose or a 'captive ball' race in each; the top steering stem nut is fitted to more than just a few threads on the stem; a picture of the rear edges of the lower yoke around where it contacts the frame steering stops?
The top yoke was used both on the '73 and '74 T150 and '73-'78 twins.
horrible notchy feel to the steering head bearings,
If they're balls, new 'captive' ball races are available under part number 97-4034; wa-aa-ay easier than loose balls.
repair the hacked tube, clean all the old paint and surface rust off, prime and paint.
It might not be obvious but the steering stops are detachable, they simply unscrew.
tank (probably a replacement),
don't know if it's the original paint colour or not.
"Redmoggy" Rod posted above that it isn't.
"replacement" (tank) is a double-edged sword - an original will cost a fortune and probably need work in addition to new paint; new can be had from India but you need to pick your metal-basher with great or the tank can need a lot of expensive remedial work here in GB to make it fit. What's up with the one you have?
going to replace it with a new harness
If you must buy off-the-shelf, I recommend buying from TMS
who supply harnesses made by Autosparks
here in GB.
However, if you use the Forum Advanced Search, you'll see I prefer diy, using components mainly from Autosparks and Vehicle Wiring Products
Main reason is even a TMS/Autosparks harness will be pretty-much an exact replica of whatever Lucas made in the year you order (e.g. '68?), so it'll have connections for separate rectifier and Zener, points, coils, condensers, yadda, yadda. However, we're half-a-century on - electronic ignition will be better value than searching out points and condensers; the existing Zener might
work but, if it doesn't, you'll replace it and the rectifier with a modern combined electronic regulator/rectifier; high-output 3-phase alternator is more 'bang for your buck', and would need rectifier 'n' Zener replacing anyway; etc.; etc.
You have to modify any off-the shelf-harness for any upgrades you want, and most (all?) upgrades leave unused wires and terminals that you have to tape up.
Otoh, if you diy, you have what you want, that can be upgraded easily at any time.
While we're talking about electrics, beware of anything new labelled "(Genuine) Lucas"; the maker has absolutely nothing whatsoever to do with the Lucas that made your bike's original electrical components. It's actually a company called Wassell, who've been making (generally poor-quality) pattern parts for decades; five years ago, they secured a deal with the current "Lucas" brand owner to use the name and packaging on the bits they've made for years.
fork legs are actually in really good nick.
there's no more than a couple of spots of light surface corrosion here and there.
What are your pictures of then? The exposed area on both stanchions between the lower yoke and the slider is badly rusted and the darker area of rust on the 'non-disc' stanchion is where the seal has rubbed over grit/road debris stuck to the oil; an educated guess says the disc stanchion has a similar worn area?
They are "salvageable" - A.M. Philpot
will do 'em, but it won't be cheap ...
New stanchions are available but can be a tin of worms - cheap can be poorly-chromed and/or incorrect diameters; if you decide to buy new, familiarise yourself with the dimensions in the workshop manual and check what you buy with a micrometer or calipers.
When you rebuild the forks:-
. ime and mho use the '78-on Super Seals (aka Leakproof seals) with the '78/'79 interference-fit retention washers, available as a set under 97-7010;
. replace the "Rubber ring" (under the bottom yoke) and the "Scraper sleeve" (on each slider) with Norton Commando gaiters specifically by Andover Norton:
. use proper fork oil instead of the recommended ATF.
brake caliper itself is a Lockheed unit which as expected is thoroughly seized, but I will take that apart and assess it,
Unless caliper and master cylinder were left disconnected from each other for a long time, you could well find the caliper pistons aren't seized - they might come out with compressed air but be aware, if they do, they come out withe a hell of a bang - maybe wrap the caliper in an old towel if trying this?
An educated guess says the master cylinder is seized in the handlebar mounting ... "They all do that, sir."
However, if you can get it apart (you might have to spark-erode the grub-screw in the mounting that locks the cylinder in position) and the piston isn't corroded, new pattern stainless cylinders are available; otoh, if none of it's salvageable, complete pattern stainless cylinder/pistons in new mountings are available.
New stainless or anodised ally caliper slave pistons are available. If the caliper is too far gone, ally replacements are available from AP Racing and patterns from Grimeca and Wassell. Note the Grimecas are 'metricated' - M10 x 1 threads instead of original 3/8"UNF, 41 mm. OD slave pistons instead of 1-5/8" OD, etc.
Connecting master cylinder and caliper, I strongly advise against simply renewing the standard mixture of rubber hoses and steel pipes; braided hose with stainless end fittings is fa-aa-ar more durable and you can make what you want yourself;
the hose can be black-plastic covered so it looks not-dissimilar to the rubber hoses to a casual glance. It's also wise to incorporate a proper pressure switch to operate the brake lamp, rather than the Heath Robinson device buried in the master cylinder mounting.