The I3 and its gearbox could also replace the dif, just like when it was in the I3 car originally, and you just need to attachit to the subframe, cut a hole in the body near the tank and its done.. But then I fear you are a just getting an I3 with a different body at the end of the process.
So maybe retaining the manual transmission will give the extra power and torque performance worthy of the RX8 , and I should mate the I3 engine (50kg) directly to that gearbox. Only problem is that rx8 gearbox has a funny conical shaft and is problematic to mate the I3 motor directly without some sort of universal adaptor...