I sugest blk02si's sugestion with a different ratio in the diff as well.
RX-7 is a tuner car, and there should be loads of used parts for it awailable if you search around. Perhaps at the scrap yard? (no phun intended).
I have salvaged lots of goodies from scrap yards over the years
BTW: You've made me all possessed about building a electric motorcycle... *shame on you* hehe...