Stuart:
“"13V" based on ...?”

Nothing terribly precise, only that it is a figure a little above battery voltage, so illustrating that the generated supply is exceeding the demand of lights and ignition (I guesstimate ~8A), which even the standard alternator should do (when new) at 3000 rpm (spec’d as 9A minimum).

My full post was:
“Well the stator and reg/rec will be new, so assuming the wiring is good, you will see some charging at some rpm. Hopefully the rotor still has reasonable magnetic strength, that will be the remaining uncertainty.
If the system voltage is at least 13V at 3000 rpm with the headlight on, I think you'll be ok.”

This makes it clear that the only remaining uncertainty was the rotor magnetism (which is highly unlikely to be as good as when new) but if the parameters in my last sentence were met, then the OP should be ok.
It seemed to be clear, straightforward English to me.

I would hope that the higher output RM23 will result in rather more than 13V in that test (with its supposedly extra ~3A at those rpm), but some of that anticipated extra may be negated by some loss of rotor magnetism. Hence my suggestion to actually test it.

Regards.