A grommet will probably do the trick. There are metal ones which permanently attach. There are rubber ones used in electrical boxes. Buy the correct size and they pop right in.
[img]http://www.hvccycle.com/images/hvc20083.JPG[/img]
You might find you have to switch your cymbal stand over from the plastic top to just having the old fashioned washer. I've got a couple of plastic tops which I cut down to just being like a washer. Then a bottom felt on that, then the cymbal with the rubber grommet. I don't bother with a top felt or wing nut at all. Looks like this in use:
[img]http://black.net.nz/drums/jazetop.JPG[/img]
You might also consider using some JBWeld on the crack at the same time.
You might also consider drilling out just past what looks to you like the end of the crack. That's a more invasive step.
Don't overtighten the wing nut so that your cymbal is constricted in its movement. It needs to swing freely.
In my database small mounting hole cracks (or "hairlines") drops the price of a 20" 602 ride to $70-$120 (based on tracking all eBay sales since 2006). Without photos or more info (like does it buzz?) yours would probably be in that range. If it was an Istanbul K you would want to do lots of work to preserve it. If it is a plain old garden variety 602 20" then I wouldn't pay too much, nor put too much into fixing it. You can get one without a crack for under $180 on eBay if you bide your time.