There are many engines that will go in but most if not all require a lot of work.
With an engine change you most of the time need a gearbox change ect.
I know of a K series conversion, an others inc a Ford Z-TEC.
If you want further info please let me know.
Rob
Triumph Spitfire MKIV 1971.
Full stage 2 spec engine.
Just completed a full restoration.