The obvious choice is this section's title

Just my two cents, as I have seen a guy relentlessly promoting "friction drive" systems - chains or belts are the obvious choice if you have anything besides the perfect environment for riding a bike (in which case, you don't need the engine!). The disadvantage to friction is clear - if you have anything besides flat dry roads and smooth tires, the friction drive will either be useless or eat your tires. If friction drive was the way to go, your bike's pedals would be pushing a friction roller to the rear wheel - but it doesn't, because friction drive just doesn't make sense for realistic biking. I grew up in a neighborhood with hills, rain, and a mountain bike. Without a chain, you're not going anywhere. And where I live now is even more full of hills and roads that take a long time to dry. A chain or belt is a direct coupling of the pushing force to the rear wheel, whether it be your feet or a gas engine, and thats the most efficient you can get. Chain/belt wins over friction hands-down in my book.

Also I'm interested if anyone has fitted a belt drive to any of these eBay engine kits? All I've seen are pretty identical minus the paint on the drive sprocket and engine color. And sometimes a pretty red cover over the air cleaner instead of the black one. But they are all identically chain driven.

This may also be a good place to ask - are there half-links for the supplied chain?