The Honda GX160 or GX200 are both great choices and available on eBay. There are also a number of Chinese made clones of the GX200 engine and these are so cheap in price on eBay they might be cheaper than used.
Both the clones or the original GX200 engines have an unlimited number of hop up parts. With 6.5HP stock they can be made to produce twice that with some tuning.
