My mate is currently dropping a warm 308 and 5spd into his bugeye 2dr, its' going to be a road-legal setup too. Should be good for low 13s, high 12s, with the size of the rubber he's aiming to put on the back!
Seriously though, the choice is yours, and only limited by budget. If you just want to spend a few hundred (taking into account you can do minor work yoself) then I'd suggest a worked 4K or 5K, they can be made to go hard, and would simply bolt up.
If you're looking for more HP, the cost is just going to rocket, as any more HP than the above, and you'd need to be looking at getting a rear end to handle the power.. as well as brakes, etc. Any 4AGE is an easy swap, gotta mess round with the dizzy location etc if you're looking to 20V it, which can be a pain, probably necessitating the installation of a tuneable ECU..
If you don't mind rotaries, an early 12A would be a simple conversion, you can buy mount kits for it, as lots of people have done this before. The good thing about going with an early 12A, is that being carb, you won't need to bother with engine management, etc, and the 4-ports start with around 130hp, which is slightly more than an early bluetop 4age, which would be a lot of fun in a wee KP!
But if you want to be "different", the sky is the limit with engine choice, I've seen everything from 308's, 350 chevs, 13B turbos, to B16A vtec motors installed into KP60's.. just have a think about what you want to achieve, do some research (perhaps have a peruse through club-k) and get some idea of what each would cost.. and go from there!

Daily driver: Toyota RunX/Toyota Caldina
Ex: 2x AE101, 5x KP60, KP61, EP71, 3x KE70, KE72, AE70, AE82, 2x TE71, AE90, AE92, ST170, plus 11 Hondas, 12 Nissans, 6 Fords, 4 Mazdas, 3 Mitsis, an Isuzu and a Lada!
