It's indeed from an old weedeater brand trimmer with the all enclosing aluminum case. My mag has the notation "reverse rotation" on it. Keeping the prop on should not be a problem. look closely -- if your's is like mine, it has right-hand threads on the shaft! The flywheel has the same Phelon P/N as one of my other normally-rotating Ryobi engines. As Kraut pointed out the fins are curved/angled the same way as for normal rotation, and given the P/N is the same, I'm pretty sure if you put a Phelon coil from a Ryobi on it (yes it fits fine) it'll turn CCW. The back shaft side casting is also a pretty good attachment point.

I haven't checked the lateral balance of mine, but it might mount nicely with the jug sticking out one side and the carb out the other. Now for that muffler mod....