conversion to RPCs than it is to find alternators. Alternators should be
driven at a constant speed, regardless of load - not an easy thing.

Not all the hard. There are governors on industrial engines that do
it well. Even the consumer quality generators do a pretty decent job
of it.

Alternators need a separate source of direct current for magnetic
excitation - also adding to the complexity.

Modern brushless alternators have the excitor built in and will
be self starting and self exciting. Some require an external
regulator which uses a tiny amount of generated AC to power the field.
For example the field on my 15KW unit is 30VDC at 1 amp. This
tiny control voltage is produced by the bassler regulator which
is powered by the alternator itself.