If you have a honda 6.5, it should be a GX200 model, unless its a clone, either way it's not a terrible way to go since you can push 20+ hp with the right parts and not mess with the bore at all. I also considered doing this with my spiderbox, but they are right, unless you want a single speed lug machine, you would need a TAV add on, or a 2 speed cent. clutch jackshaft for more speed and gearing options, but the cost of that alone would far exceed picking up a used 150-250cc scooter engine and essentially having a bolt in power plant with the option to build it up further.
Then again, if you are feeling crazy, you can look for a quad or dirtbike engine and throw on there, but that takes a great deal of time and fabrication, x-bird and I both will tell you that.
For anyone wondering about the 2 speed jackshaft, I will find the link for anyone interested. It was popular on some older mini bikes.