My vote is...
A lot of money.
A KA puts out about 150 or 160 stock... it would take a bit of cash to turn that into 200+ horsepower. I think you'd be hard pressed to make a 200 hp L20B. I think you'd be hard pressed to make a 150 hp L-Series. I know you said non-turbo, but maybe if you went turbo + fuel injected, maybe did some bottom end work (stroker, bored over?), did a really nice job on the head, did all the ignition, fuel system, exhaust... you'd probably put out some pretty good numbers. It'd certainly be a beast. Hell, the J-13 in my 521 only puts out 67 hp, and it's got some kick.