without a pic its kind of hard, but here is what i know. if its a shaft driven car and has a 15fe with a purple cooling head, its likely a nitro rs4 3, an early version of it. if its shaft driven with 15fe and has a head with vertical fins its may be a super nitro rally.
hpi uses long shaft engines on every touring car i can think of. you will be able to keep the clutch, clutchbell and flywheel, and depending on what motor you get, the header and tuned pipe. depending on what motor you get you may need to get a set of engine mounts, hpi likes to build them into some motors. look around at other engines, the engine exchange isnt that great a deal, you can often get engines cheaper through a hobby shop or online without the exchange, even hpi engines.