My purpose is to test outside the generator the charger board
So is it your intent to test and rebuild *original*. To keep in some sort of preserved state. Or do you simply want the charging to work.
If the later, indeed as per
@Berni simply rebuilding the charging board with off the shelf components may be easy: 3-phase rectifier, DC/DC converter, and some modern smart monitored Led-Acid battery charger circuit.
But if you want to keep original, and indeed want a test bench for your 36V 7Amp 3 phase:
I think I would start with a 230V VFD, and then run it through 3 transformers to both make it into a proper sine, as well as step down to 36V (and maybe also provide isolation from the grid)
BTW: do check if the 36V is 36V peak, or 36V rms, and whether it is star or delta, so you get the correct transformers. Unfortunately 36V is not very common off the shelf but maybe you can get some that have both 12V and 24V windings.
EDIT: also, thinking even simpler:
You can probably simply component test the charging board *without* needing all 3 phases. CHances are good the whole things starts with a 3-phase rectifier anyway, in which case you can just run and check the board with a single 36V AC power supply. If the first stage is indeed just a hextet of dioded, there is no real need for all 3 phases, except maybe the resulting voltage will be a little waivy, but even if that turns out to be important, you could temporary hook up a beefy capacitor.
You could even simply inject a good 40~50-ish Volt DC , right at the input of the rectifier, and it would pass right through and supply roughtly the same DC current as the recifier would, for testing the rest of the components. Maybe 3 or 4 small lead-acid batteries