I love RX-7 engines - they are very low maintenance and make good power. No torque, but good horsepower. The cars are light so torque is not an issue. The exhaust gas temp is extremely high on a rotary, 1900 degrees if I remember right. Take care on the exhaust routing. Very compact engines with extremely smooth pulses (no vibration to speak of.
Need to check fitment on a speedster, but I don't remember them being much longer than a flat 4. One thing - getting harder and harder to find good rotary engines. Alot of worn out ones, but not good ones are becoming more rare. Used to be that you could buy them from Japan for a couple of hundred bucks for a 12A or a 13B. No longer the case. Alot more money now. If you wanted to get crazy and go into a TT, you'd really have to be clever at planning the heat management and plumbing in the bay, but I suspect it could be done.
This is an engine I would consider for a speedster. A buddy of mine and I put one in a Fiat X19 many moons ago. Went from 67 horsepower to 180 - whoo-hoo! Way more fun! The catch is, there's quite a few other engines I would consider for a speedster including a 911, or a subie.
angela