There are a lot of mods from Airfilter to Exhaust to Gearing can you please help me with what kind of Exhaust I should put on my BikeHonda GXH50? There are very few performance mods for that engine. An NT carb + 4-stroke adapter intake will slightly improve reliability using US fuel, and replacing the stock box muffler will slightly improve the high-end and exhaust sound, but that's about it. There is an aftermarket cam sometimes available, but that won't improve the performance much. The compression is too low to make much use of it (8:1).
The head isn't removable, there are no aftermarket pistons or con rods available. Aside from an exhaust upgrade, I'd say run it as it is.
so your Brother has my Engine on whatTake a chinagirl exhaust, cut the muffler off of it, bolt header to engine, slip on a flexpipe about 30-36" long, use a bicycle seatpost clamp of proper size to secure it to header. Add muffler to the end of it. Quite simple, can't explain it better than that.
Those "lot of mods" aren't going to do much for your engine. My brother has had a GXH50 for 10 years and didn't have any problems with it until he started over-revving it and then rebuilding and modifying it. The engine is pretty good out of the box, keep RPMs under 8K for best results.