They are good engines. Very well made. OPS added a lot of cooling fins to help the engine when it is in a scale plane with a cowl all around it. So the engine works well where other engines would tend to overheat. Now it doesn't make as much power as some other 30cc engines, but it has a lot of torque to it, so it runs good in any case. The extra weight is a plus when you put it in a scale plane with a short nose on it too. I remember guys having to add extra lead weight on their planes to get the CG forward more. So with this engine you may not need to add any extra lead weight to get a plane to balance OK.

You run 0% to 5% nitro glow fuel and after you get it run in you can lower the oil content down to 12%, some buys say 5% oil works on them. If I remember they have a needle bearing rod inside, so they can run with lower oil content. It tends to like a 18x10 prop where other 30cc engines like a 18x8 prop instead.  Quite a few fellows converted their engines to spark ignition and gasoline too.

Here is a video of someone flying the OPS engine in a Stampe SV4 bipe.
http://www.youtube.com/watch?v=OLd5RRALkSs

They have the MAN article engine review  MAN June 1998, Vol 126, page 66 by Dave Gierke.
If I find the article, I'll let you know.