if you had something that you could start moving yourself and then kick the engine on for power it would work. it would never get you going from a stand still. also you need to remember that 25cc's from one company is like 15cc's from another. some brands will make a lot more power with the same amount of cc's.
Why not just use a 3.5 Briggs And Stratton and govenor it down if you dont want it to go fast... hell you can find lawnmowers on the side of the road, and some of them might be locked up, which is still fixable, or some might need a coil... hell i have found them that just had bad gas in them . Also i cool thing to do with a weedeater is but like a trolling motor blade on the end of it and use it as a boat motor since they can run upside down the only problem is water running up the tube....
chainsaw is the way to go for kids on a gocart it can be governed pretty good so it doesnt excelerate so fast and do damage to any thing especially the kids they do a real good job and they can fly when you want to ride it
It wouldn't work, we've tryed it just for the heck of it with a go kart and lil mini bike for a little kid, just do what i did and put a 440 snowmobile racing engine in a kart, mine lifts the tires off the ground. haha.