Thanks Dave, it was a long time in coming....
The switch between Motor (starting) mode & generator mode ended up coming out pretty simple.
Initially, I wanted to use the 48V batteries as a source for starting. IF I did that, though, the switching got pretty complicated from the perspective that I would need to use double pole double throw switches, and on top of that, at 48VDC it MIGHT spin the engine too fast; I.E. without it slow starting, this beat of a motor might just spin the belt really badly, out of pure torque. Like you said, that MARS Motor IS a beast, no doubt.
So....I was sitting there looking at it, and I had a car battery sitting there in my shop, too. I grabbed the battery (which was low at the time) and put a set of jumper cables on it, and tried to start the Lister with it with the cables hooked onto the (2) posts on the MARS & on the positive post on the battery, & touching the negative to see how it'd do.
The result:
It spins up the Lister slow enough to where there's no bad torque issues, easily fast enough to start, voila!
On wiring, we start out w/ (2) #1 cables coming from the batteries, the positive goes through a set of diodes (to stop the MARS motor from "running" on battery power, and the negative goes through a 150A fuse. Downrange from that, right at the motor, I set a battery, and ran the cables the same you would in a car, I.E. the negative is run direct battery to MARS, and the positive goes through a solenoid that is fired by a switch (that I can tap my foot on).
To start:
(A) Turn on Fuel on Lister
(B) Put foot on switch, MARS Motor runs, Lister spins
(C) Once the Lister spins a few times, release intake valve for compression
(D) Take foot off of switch, machine running!
It's really simple, in the end. The Lister gets started by a 12V car battery, via solenoid. once running, the power made is driven STRAIGHT at the batteries, at nice BIG current.
That's about it; the MARS is held by a frame & a regular alternator bracket.
The guy that owns EV Drives, Carl, was very helpful in getting this done.