Torque gets it going, horsepower keeps it going... those guys are gearing the small engine to the moon to get a reasonable horsepower out of it compared to the ancient gasser that made 20hp at 1600rpm. I'm not going to do the actual math, but horsepower=torque(in pound feet)*rpm/5252, so to get that horsepower at 1600rpm (it's been a while since I've looked, I think the IH A and maybe IH M are rated around that speed) it takes a lot more torque than it does at the 3600rpm small engines are rated at anymore. That said, gearing the engines down the equation does work in reverse with leverage so it can put out the torque because of the horsepower it has at the speed it is running... so a 2:1 reduction would double the torque available.
It's an interesting concept, personally I'd want to keep a standard type clutch instead of a centrifugal, and I'd be adding some muffling to those things, that was obnoxious and I wasn't even on the thing!