Need Help Read Plz!!!

ok, i was just given a 87 crx dx? i beleive, its not mpfi its carberated, i just have a few quick questions, instead of doing some crazy expensice swap, couldnt i just do a d15 duel carb swap easly? why or why not?! and whats the gas milege on me rex? 160k, runs perfect, just wanted to know about the swap!!! ty!