Don't really know about 30 yrs. worth of seizeure but I freed off a well stuck engine with heat. Penetrating oil for sure, let it sit overnight in an unheated area, the colder the better then heat the barrels with a torch as evenly as possible all the while rocking it back and forth in top gear.
If it's a complete bike,start by tying the clutch lever back to the handlebar.
Take off the primary cover and see how much rust is there.That alone could be stopping it.
Drain the water out of the gearbox (it's sure to be there),and re-fill it with penetrating oil (WD-40 etc).If the gearbox is already in gear,that could help.
Pull out sparkplugs and see how far pistons are from TDC;about mid-stroke is good.Pour some penetrating oil in the cylinders,and re-fit the plugs.
I like to see that the valves are not stuck.Some penetrating oil on the stems can help;through the rockerbox,and into the ports.If the pistons aren't close to TDC,you can bounce the valves open 1/16" or so and see if they spring back.
You might be able to detect some movement,if you put a spanner on the crank at the rotor nut.
Let us know what you find.It will be easier to pull apart or even start up,if you don't go any further than that for now.I've started up stuff that sat in a field 30 years,and had it running after unsticking valves.
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