Many people put in Rover K series engines and Ford type 9, 5 speed gearbox's. Mostly in later cars but there are a few converted Frogeyes.
Around 160bhp to 180bhp is available without going mad on cost but the half shafts tend to break.
My thoughts tend to be yes improve the A series a bit to get some more power but rather than spend £10k at Frontline Developments to update the car, just accept it is a Sprite and that an MX5 is the sensible way to go for a modern drop top and cheaper.
