So many opinions and tests etc, bet you're still trying to unravel it all!
In a nutshell, your alternator in new condition is rated at ~120W output, only delivered at high revs, say 5000 rpm. At 3000 rpm (~50 mph in top) it may only output perhaps 80-90W or so.
Your demand on the system is constant 55W headlamp, ~10W tail & sundry, ~35W ignition (not much difference EI or points) - total say 100W.
The equations don't work at those revs/speed usage, even if all the bits (alternator, rectifier, zener) are working 100%.
So if only riding <45mph in town, either do it in 2nd gear, or reduce the demand. Do you need anything more than a halogen 35W dip bulb? Use LED bulbs for tail etc.
IMHO the best solution is a 3-phase alt, with solid reg/rect, which will give the power in the rev range you are using, as it produces more max power(180W) and the output climbs in a steeper curve from tickover upwards.
3-phase uses the same alt rotor. It is unlikely that your rotor has signifiantly lost its magnetism, unless it has been seriously shocked with a hammer or stored in isolation (not inside the stator).
Even if you elect to keep the single phase alt, don't waste your time with zener/rectifier, use appropriate reg/rect box.