Ah man, I thought when I read this post I was going to find the clutch I need. I bought an edger at a yard sale for $20 not to long ago that I figured I'd use the motor for a kart. When I pulled the pulleys off it had a 7/8" shaft where the last 1/2" steps down to 5/8". Now it just sits there, basically useless to me. It starts right up and runs good though. I can only find the same $100 clutch on eBay. I may try adding a sleeve and stepping up to a 1" bore clutch.
if there's a keyway, then one could use a bore or endmill with a slow feed. however, that does leave you with a shallow remainder of a keyway that needs to be deepened to accept the new key.
i don't exactly like paying extra for the convenience, but i do enjoy "bolt-on" solutions when available...
speaking of keyways, i'd been wondering about adding an adapter sleeve and the gap that would be left...smallenginewarehouse (on ebay) offers this kit w/ 3/16 - 1/4 adapter key, which looks pretty handy, but still it exceeds $20.