I would have put this in an austin-healey forum, but there isnt one on this site and the midget was originally a sprite anyway (I think) has anyone heard of taking a modern motorcycle engine and putting it into a sprite or midget?

Most motorcycle engines have the transmission combined with the engine, The first question you should ask is how will I back the car, motorcycles don't normally have reverse. Motorcycle engines would need more rear gearing than a Midget/Sprite to keep the engine in the torque band. Never heard such conversion. There are a few Spitfires with Mazda Rotary engines but that may not work in a Sprite bcause the engine bay is narrow.

Clifton