It's been a while since I've worked with simulators, and it was a much older generation of sims than what we're using now. I don't know if it would be as simple as pushing a button to do the software load for the Max training, then reloading the standard NG software to restore it to the original, approved design.

I don't think that hybrid Max-in-an-NG software would be usable for anything except the MCAS/runaway trim training, since it wouldn't 100% replicate any specific airplane type. But to get the crews trained on the differences and able to fly the plane, I think it'd be a very reasonable, realistic option, if the FAA is on board with it.