Mike,
I would serioulsy reach out to one of the local RX7 shops. The "E" shaft and the cooling are critical in a three and even more so in a quad rotor configuration. Plus that monster would be loooooong. Make sure you don't have to extend the front of the car to get it to fit (Or sit in the back seat). The RX8 has a shorter snout than the RX7.
Local shops
Defined Auto works (Logan Carswell)
Zavier (Was RX7 Store now.......)
Carswell Rotary (Logan's dad)
Others??
Not saying it isn't possible, but it will be stinking expensive.. 30 K for a three rotor sounded more of the norm and using a Cosmo 3 rotor.
Plus the decision about turbo / not has to be made before you build the motor (High compression or lower compression dorito's / Rotors).