I don't know that anyone really sells a "bad" engine anymore. What's nice about the O.S. engines (the non-ringed models) is they require very little breaking in before they are running and idling reliably so you can just go flying and not worry about tuning all the time.

With less expensive brands, they may require a little more break-in time (my GMS .47 and Magnum .52A engines come to mind) or they may need a little more maintenence. Bolts may work loose on the cylinder head (my GMS .47) or you may need to lok-tite the muffler screws or fix an air leak in the remote needle valve (my Magnum .52A). Cheap engines will run great, too. They may just take a little more time and effort to get there.

O.S. Max 2-strokes break in very quickly and everything stays pretty well bolted together without requiring a lot of fuss. Particularly in the .40/.46 and the .61 size two-strokes, however, I don't think you can buy a "bad" engine from any North American distributor anymore. O.S., Thunder Tiger, Magnum, Super Tigre, MECOA, Evolution, Webra, GMS, KangkeUSA SK and JBA, Norvel, Aviastar, K&B, HB, HP are all good running, reliable engines in this size range. Each brand has strengths and idiosyncricies (fuel prefrences, prop ranges, break-in periods, glow plug ranges, etc.), but all of them will provide good reliable power if you break them in properly and spend a little time to figure out how to best tune them up.