is how get the alignment correct. I can drill a hole in a plate of
aluminum for the PSRU, but how can I then hold that hole concentric to the
engine's output shaft while I do the match drilling? Alignment is
critical, as any offset will vibrate my teeth out and will eventually
destroy the coupling.

Any ideas would be greatly appreciated, as I think I'm going to be down to
spending money at the machine shop for this one.

Excuse my ignorance, but are you drilling clearance holes for the bolts?
If so, I would drill the hole for the output shaft first. Then, I would put
transfer screws into the motor to mark the bolt holes.