Pictures Please!!!

You should be able to get a YS check valve from YS Parts and Service (pn: YS0405)

http://www.ysengines.net/support/parts/

Since you have it apart I would (personally) put a set of bearings in it...

A little problem with the YS60 is that the gasket sets are not available any more so you will more than likely have to make your own, unless you can find someone has a set laying around...Not that big of a deal though...Good thing is the diaphragm, spring, plunger, needle valve/seat and carb o-rings are still available...I'd personally order a set of those and replace everything while I had it tore down...

I attached a copy of the manual in case you need it...

Good Luck with your Atlanta!!!