1993 Honda Civic Engine swap (D16Z6 to D15B7 to D15Z1)

I have a 93 Civic EX (originally D16Z6) for which the previous owner had to replace the engine and put a D15B7 but he never used an air filter so it's giving an awful mileage (21-23 MPG) and really low power, hard starting in the morning (engine stalls about 3-4 times before staying on and doesn't stall anymore afterwards unless turn it off, for which I blame the piston rings 'cause 1 or 2 plugs are oil wet), smells like gas in the engine compartment but no visible leaks (internal maybe?) And like burned oil in the passenger compartment (this is a really hard one, no wonder I had to pay nearly twice as much for registration than for the car itself), and since I'm going back to school I will need to get the best mileage possible with it so I'm thinking on installing a D15Z1 on it but I was just wondering besides the engine and ECU, what else would I need to replace on it if it can actually work for me of course?