as a former RX-7 owner (10th Anniversary Edition Turbo II), I'd say you'd likely need to replace the whole motor. Try the ATF trick first (google it) and that can sometimes free up the rotors if the engine is seized due to carbon lock. If you do decide to replace it, don't even consider a non-turbo. Rotary engines require a ton of maintenence, and they're expensive to work on, I basically taught myself to rebuild them because there were so few mechanics around. I'm honestly a fan of the old RX-350 build up, might even do an FB build. A V8 swap offers much more bang for your buck as long as you don't mind sacrificing the high redline and handling.