Nothing new under the sun, Mitch. 40 years ago we were putting McCullough MC-5 and MC-10 chainsaw engines on go-karts and they would flat scream and fly. However, they were a tad-bit tempermental. If you had one set up right though, it was generally capable of winning the race.
However, for slow and reliable go-karts, I agree, stick with Briggs and Stratton or Tecumseh. The gearing is the secret. Calculate from the max RPM through the sprocket ratios to tire surface feet per minute to MPH. Allow 30% for clutch and tire slippage to get the speed you want the kart to go. After all this, I have seen karts advertised in the $500 range and when you start figuring all the part costs, let alone the cost for running around to get them, you might be better to start with a purchased kart.