I'm assuming you want a small sprocket for the engine? If so, maybe you could buy a #41 sprocket with an undersized bore, then drill or machine it out to match your engine shaft. Or deliberately purchase one with a larger bore than you need and make a bushing to go inside. Or buy a sprocket without a hub and have it made or machine it yourself.
There's several ways to solve this problem, depending what you have access to, what you want to spend, and what is convenient.
While I think selling karts like this on ebay, mail order and through retail outlets would be an amazing thing, I personally would be terrified of the potential liability.
About five years go I cobbled together a CNC-controlled plasma cutting machine to mass produce roosters made out of 22 ga mild steel for some guy I met on the internet who was cutting them out by hand with tin snips and couldn't keep up with his customer demands.
Anyway, I had quite a few screwups left over that I wasn't going to send him but they looked good enough to sell as a single, and I did just that on ebay. A month after I sold one, I got a nasty letter from a lawyer out in Iowa because his client bought one of my metal roosters and cut his finger on it pretty bad, to the point where he needed orthoscopic surgery to gain use of his finger back.
My response was "read the text on the auction which is binding and legal according to ebay - sold as-is, no warranty or liability of any kind assumed, implied, or offered.
So if some putz can almost bleed to death after puchasing a weather vane I can only imagine the liability of someone buying a fancy, quick kart like what you're making T-man.
I mean by all means go for it, but I would find an attorney that could help you draft up a no-liability document of some kind and include it with every sale just to protect yourself.
