The obvious suggestion is a chevy small block. Dirt cheap and with so many speed parts available as to make one's head spin. Tight squeeze into a sprite, tho. (As you proboably know, there were more than a few 3000's converted to v8's). I'd also be concerned about weight & cooling.
The old 215 v8 is an interesting option, the Brit car folks know that as the motor from the TR8, Rover SD1, Land Rover Discovery, etc., and at least one person has done it (
Spridgets ). Expensive, though, and not easy to come by all the parts you might need.
IMHO, the best options are either a Japanese motor (e.g.
Toyota Egine Swap ) or a rotary. I'd prefer a newer Japanese 4 cyl motor, but that's not all that easy because many are fwd. The rotary has been done in a midget:
MG12a, Rotory powered Midget
Actually, what I would probably do is restore the Sprite to the best condition possible, sell it and buy something else. Even with the better engine, you're stuck with suspension and other features that would make me really question whether it was worth the effort & $$$.