This site has a sub-forum that is for gas engines, but it's not very good really.
Check out motoredbikes.com or motorbicycling.com; they're pretty much about only gas-engined bicycles.
The China 2-stroke kits are very cheap and have some issues right out of the box.
I had a gas-engined bicycle for a while just for fun; it was entertaining but had some big drawbacks as well (even tho it was one of the more expensive kits out there). It was not one of the China kits, nor was it a 2-stroke engine.
The setup that works best is the Staton/NuVinci one, because the engine drives the rear wheel through the NuVinci variable-ratio hub. These little engines make VERY little torque until they get pretty high up into their 7000-RPM ranges, and so they really need a variable-ratio transmission to work through. If you try to start up a slight hill or into a 15-mph wind, they can't do it. They don't have enough torque at low RPMs.
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The NuVinci kit is the only one that has that feature--and they don't sell it any more. I'm not sure of the reason they stopped, you'd have to ask on the forums above. I know that it was the most-expensive gas-engine kit there was, when they were selling it. NuVinci hubs are still available for $250 and $350 but I don't know what other parts were involved.
If you have hills and you don't have to go far, then you'd probably do better with an electric set-up. Electric motors make their maximum torque at zero RPM, so they have LOTS of hill-climbing torque already. What they don't have is long-distance endurance, and buying big high-capacity batteries to help that ends up costing lots and lots of money. Getting 10~15 miles out of a decent battery pack is not impossible tho.
I think that gas engines can be way cheaper and more useful (just like with cars!) but the engineering required is a lot more complicated than it is for adding an electric motor to a bicycle.
As for "bicycles don't have motors!", well, some people are old and crabby and have no imagination.
Just because they live a bitter and humdrum life doesn't mean you have to.