IMO: sell the 18hp. Buy a comet tav2 torque converter, live axle and necessary components, and get a free flowing header and filter for your 5.5hp. Is it a honda (google).
if you want to mod it a bit more, I would sell your Honda and buy a gx200 clone. 34cc extra displacement to play with. You could install a flat top piston and a bp-2 (or a mod2) cam from NR racing, arc racing or affordable go karts. You can get approx 9-10hp after spending $100 on parts.