The 1275 fitted to these cars originally is the British equivalent of the small block chevy here, there are plentiful stock and hop up parts available, and they are very nice engines in stock form, but also very tunable. If you want to swap several options have been mentioned, most involve a little cutting or metal work, another one worth mentioning is an early RX7 rotary, which is nice because it is light and doesn't upset the handling.