As far as I know Hobbi o is a distributor for OS and doesn't actually own them. I'd assume OS, like other engine manufacturers, does make engines "to spec" for other manufacturers. Now a "to spec" engine can be the same as an engine in their normal lineup but with a different cooling head, different porting, or different materials and machining processes to lower costs of the manufacturer contracting OS wants a lower spec. If a particular "to spec" engine is not offered in OS normal product lineup then Hobbico and other distributors wouldn't have any info on it. SH makes engines "to spec" for LRP, Losi, Redcat and others.

Both OS and SH make good engines, but what Redcat spec'd for each is unknown. Ultimately I think your decision is going to have to be based on how bad you want the electric starter. Removing and installing the flywheel isn't hard. You simply hold the flywheel (a tool like a pair of channel locks may be needed), remove the flywheel nut and tap the flywheel lightly around the backside to break it loose. Don't forget to remove and save the collet on the crankshaft behind the flywheel.