Yes, GMS does make the .46 and larger engines for Tower. Personally, I'd rather buy from GMS and not have to fool with those ridiculous remote needle valves Tower uses.

While a few folks have had trouble with the GMs engines (usually running even after throttle completely closed, due to defective carb or front bearing) RJL, the US importer, has been very good about making corrections, from all I've heard.

And I hate to disagree, but I know of LOTS of problems with the O.S. FX engines, due to defective cylinder liners. There are better brands for less, or equal, $$. Just my opinion, of course. Buy what you like.