Regarding rafts of engine concepts, a vehicle with electric motors and an empty engine bay could offer a myriad of possibilities. Just add electricity.
Xoon mentioned free piston linear generators. A few of these, and whatever you want to feed them in the tank, and you can go do your thing. Additionally, if the vehicle were to take a hit, as long as some FPLGs remain operational, you do too (I think).
A group of students from Eindhoven built a formic acid fuelled range extender trolley to hang behind an electric city bus. If the contents of the trolley could be made to fit, that would be a pretty sweet means of electricity generation. It produces far less waste heat and noise than a combustion powered generator. Unlike petroleum-based fuels formic acid isn't flammable, and when you do it right it has no carbon footprint. But it's a big if. This approach assumes a rather high efficiency compared to fossil fuel systems.
These two articles suggest other compounds as hydrogen carriers, most interestingly carbohydrates. The latter article suggests as much as 10 MJ of H2 power from 1 kg of carbs, about twice as much as formic acid's 5.22 MJ/kg. But then again, I've seen the formic acid trolley on TV, but no sugar car so far. Maybe it's a good next step after you've bought your formic acid powered tanks. They have electric motors already.