A 10x6 on an OS40 LA will not hurt it. I got into the sport back in Jr. High with my Dad. We both learned on a superstar select (os 40 LA). We've always ran that engine with a 10x6, and its performed great. Infact it still runs fairly well, and I just finished my freshman year at Texas A&M. Now givin we don't fly every single weekend, but that engine has survived everything. From crashes, a event our club used to have called junk yard wars (yes, like the old TV show), and general flight. The key thing is to make sure that you run them a little rich. In fact, I believe a 10x6 is what they recommend.

Forgot to say, if you want a new engine, I would look at a 46 size Ball bearing motor. I personally like the Tower 46. It gives the profile in my avatar unlimited vertical. Plus it gives a trainer a whole bunch of kick.