I use these guys:
http://worldwidebearings.com
They have been great and are very knowledgable, used to supply GGNA and Ty Davis's team. The "C" is the clearance spec, which is always C3 or C4 for a main bering in a two stroke application. C3 is good. You could actually go different ways with this: the OEM parts, the newer sealed ball bearing replacements, or heavy duty M series open ball bearings. Worldwide is doing a lot of ceramic stuff now too. See what they say, but OEM is fine. I'm not sold on the sealed ball bearing idea yet.