The questions youre asking make me think you may not have the capabilities to safely build something like this yourself. Once you get into motorcycle engines, your really leaving the realm of simple yard carts. hopefully that doesnt come across offensively. just some things to consider
-Do you plan on making this chassis yourself?
-If not, consider most go carts will need serious work to handle the power...been there done that...wasnt worth it. Think front brakes, custom engine cradle (cutting torches, or a LONG time with an angle grinder), shifting linkage, reinforced rear axle carriers, upgraded tires, steering components, etc.
-A single cylinder 4 stroke air cooled motorcycle engine really limits you, and in my opinion is a lot of effort/cost for not much reward. Most single cyl air cooled bikes are dirtbikes, which hold their value fairly well. my 350 suzuki, makes 33hp, but the bike even thrashed would still be worth 800 bucks.
The cheapest motorcycle engine you will find, is likely going to be a 70s/80s honda yamaha, suzuki or kawi cruiser, 2/4 cylinders, and likely 400cc+ I've bought many complete motorcycles in the 50-200 dollar range (im up in vermont, some even came from mass). Is there a specific reason you want single cylinder?
-An ATV engine may be better in some scenarios, as it will likely have reverse. But most air cooled ATVs are relatively low horsepower/displacement.
