Just get some emery paper and start up the engine and sand it down till a 3/4" clutch fits. 20mm clutches are pricey. U only need to sand it down a tiny weenie bit for it to fit so do it slowly.
I have a honda wet clutch 2:1 gear reduction unit that fits a 20mm shaft http://www.diygokarts.com/vb/showthread.php?t=4219 you would need to deal with the 22mm output shaft on it though but a 22mm sprocket is much easier to find than a 20mm clutch
Sanding down a spinning shaft is not an accurate way of making metal parts fit together, even if one wraps the emery cloth around a metal plate or thick block of wood for rigidity. It's very easy to "skew" things and taper the shaft accidentally.
I would take the crankshaft out of the engine and turn the shaft down on the lathe, or puchase the correctly sized clutch for that shaft. If neither are in your price range you could overdrill the clutch's hub and install a sleeve you buy from mcmaster-carr or another place that sells similar products.
If you're going to try to turn your shaft down I'd suggest putting a long bolt in the case and use that as a rest for a flat file then run the engine at idle and go very slowly. This wasn't my idea but I like it.
I've done this before "in a pinch" for non-kart purposes and never got the shaft straight, even with my making a simple rest. Though, I had to take off more than 3/100th of an inch... almost 1/8", hence why I warn against doing this.
if you keep the file where it is just barely contacting the shaft it should stay round, if you put pressure on it you'll likely end up with a low spot around the keyway because it would dig into the gap. Make sure and wear a face shield or at least safety glasses.
I will make up a sleeve on the lathe that will go over the crankshaft and then it will step down smaller so the cluch can fit on and this will have key ways and grub screws it it.
or u could make up a mount for the engine on ur lathe, run the engine then slowly cut it. Use the engine as the lathes engines and just move the cutter peices into the shaft. Type GX200 shaft down size or something on youtube, some guy did it.
i have a brand new robin subaru EX17 6 hp power source for my kart and i can not sand it down the warranty will no longer cover for what happens to the engine please help me some one
i have a brand new robin subaru EX17 6 hp power source for my kart and i can not sand it down the warranty will no longer cover for what happens to the engine please help me some one