What is the fastest engine that will fit in my civic?

i have a 99 civic lx which i want to swap. i have had the d16 engine for a while and it has 150k+ miles on it. i will be keeping it over the next 3-4 years at which point i will swap the engine in place of buying a new car (seeing as how i have put a decent amount of time and money into visual mods...not too ricey)

i would like to put in a k20a, but i wasnt sure if it will fit. if it wont, what is the fastest engine that will fit in the bay?

also, it is currently automatic and at the same time im swapping the engine i want to do a tranny conversion. any tips/estimates? (how much for the tranny, how much for the engine, and the labor for both)

any help would be much appreciated

im not sure about prices, but you could put in a turbocharged rsx-s engine, it think its called the r22 or somthing like that
about the best you'll do its about 250 horses