First you need to make it last, so you need Billet rod and flywheel,flywheel could be optional if you have a real honda flywheel not a clone but i would get the flywheel, then for power, get a flat top piston with a cam around 240 dur keep lift around .300, the 260 dur makes more power but if you use the 260 you will need a valve train, rocker arms,pushrods dual springs, etc.,if 240 dur use heavy duty springs, do a mild port clean up on intake side, I like stainless valves, but honda will work, lastly you will need a carb to feed it, 30-34mm range.
This should get you solidly around 20- 25 hp, There are others who may not agree but I have build the above engine, I haven't dyno it yet,still looking for one near by, it feels solidly in the 25hp (seat of the pants) range. I also have the 260 dur engine and it feels like 35+ hp, but it has good amount of head work done to it and a pumper carb on methanol.
Hope this helps