Heheh One of Hebe's rear bump stops is held on with cable ties.
It really depends on what you want out of your engine and your car. Different engines are known for different qualities. Of course, whatever happens you'll need to upgrade the brakes and suspension.
The K series is a common conversion and has been proven a few times now

The Twin Cam from fiat is also very popular and you can still occasionally stumble across a good one or two.
If you're wanting to go way over the top I've seen a few v8 rovers fitted in (or rather, a few minors fitted around a v8:) ) and someone once even put electric motors in their minor
Whatever you do put in though I'd strongly advise looking at the whole picture rather than just the power plant
