If you are going to have AC power
available, why not ditch the DC power nonsense and go with a variable
AC motor speed control and a suitable AC drive motor straight off the
alternator, making it all more efficient. There are some whopping
variable-speed AC motors available from electrical supply places.
746W/horsepower, you'd have about 6 hp off a 5KW genset at full
throttle. It'd be a long road home.....

A very old 6-71 has just been broken in if someone changed its oil
when they were supposed to. Is it hard to start in its condition?

Yes, hard to start unless preheated or shot with ether. Runs OK once started.
OLD... ex navy 1944. Estimate over 20,000 hrs w/o OH. Will have to consider AC
drive motor idea. Doesnt speed control involve a $$$ variable freq inverter?
Or are you talking about brush type motors?