" the entire powerplant has to fit inside a 20" box"
boggen, do a goggle search and chase down "Tesla's bladeless turbine", it's the only option small enough to fit and produce the power you need. It can be run as internal combustion OR steam driven. It can be made small enough to fit at each wheel, plumbing them would your task but 100psi would not be a problem.
Good luck with it.