How much are you willing to spend?
4a-ge `Silver Top` or `Black Top` is the usual conversion, a supercharged 4 a-gze is a bit exotic, but Nissan`s RB20DET will go in it.
Stick AE86 front suspension and brakes on it first though!
I`m sure there are other engines that can be made to fit too.
Great little cars.
Edit: DrkLce is right Mazda rotaries will go in too, there are a few about and they make a lovely noise too!
www.motorfix.jp