Full wave rectified 60 Hz AC is pretty noisy power and generators are notorious for making very spiky power on top of that.
MPPT controllers deal with "shifts" in voltage and current from the panels, but usually they are not intended to deal with things that happen at 120+ Hz, which is what you are asking it to do.
I suspect that in order to make the rectified power clean enough to feed into the classic, (without risking damaging it), it would take at least some amount of capacitance and inductive filtering to be sure of not damaging it. By the time you do that, it might be 1/2 way to a DC charger.
I know that you are trying to help them save money, but supposed that Classic dies for some random reason? Even if your idea had nothing to do with it, people will wonder if your modification caused their $800 classic to die and at least in their heart, hold you accountable.
An IOTA 24VDC, 40 amp charger is around $400. That is 50% of the cost of a Classic if it had to be replaced.
Another path, that might even turn out to be cheaper is to remove the AC generator head from the diesel generator and replace it with a 24 volt alternator. 24 volt alternators are common in heavy duty trucks, some school buses, and the marine industry. Marine industry equipment tends to be expensive, but semi trucks are very common so there are a lot of parts around.
Some of the high output 24 volt alternators use an external voltage regulator, but I think some are built in.
100 amps from a 24 volt alternator is easy - as long as you achieve a minimum rpm. Below that minimum, they more of less generator zero power, but the engine does not have to run at a constant frequency like an AC generator.