Wow. Went to Harbor Freight to see the motor firsthand... tiny little bugger! Its 79cc displacement, and it is one of the OHV types .............. tried and true techniques to the little guys should yield 4 - 4.5 hp without any expensive parts..
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4HP+ would be nice, as I wasn't expecting that much.
This engine is NOT a clone at all, but it's own model and size. It looks to be designed after the GX200 clones, but really shrinked down to 79cc, because the Valve cover is huge, and so is the pull-starter. The ports are short and wide on these, but look like porting sure would help.
I also like how the stock header on these unbolts from the muffler, so you can attach your own exhaust to it.
The carb is not very big, and I think that's where most of the restriction is, so it's only 2.5HP. I wonder if a GX200 carb would fit. Getting 4HP out of something this small, and still having the low-end torque, and no-oil mixing of a 4 stroke would be great.
I also notice, this 79cc engine is more tolerant to high revs and backfires less without the governor than a GX200, and floats the valves less, but they still float some when the RPMs max out. I imagine some springs could be found for these engines.
You can also feel the air rushing into the vent with your hand, which to me shows the restriction. Go to Harbor Freight to see for yourself and check out the 2.5HP engine. When you see the stock air inlet, you would laugh. 
), Make or somehow acquire a stepped header that fits it, and I might do a little smoothing and de-burring of the ports. All of this can be done for very little cost, and if you tune it right, this may get you to around 4hp.
