There's a lot of information I've gotten off of
Drag Racing 1964 Electric Chevelle. He uses two or three contactors to build a multi speed "controller" that switched the circuit from parallel to series the same way the killacycle does for a cheap drag racing EV. If you want to go cheap, but hopefully avoid the whiplash Ben's talking about, it'd be worth a look. I'd be surprised that a 5hp motor would do much whipping but I have zero experience. He was also only using 72V, but had a much larger engine.
What kind of AC electric motor is it? I remember there being a general purpose cheap motor type, used for fans usually, that doesn't care whether you feed if AC or DC, but are horribly inefficient. Then there is also the brushless inductive motors that obviously will do nothing if you give them DC. Does any of that make sense at all?