Dennis, it's common for beginner's to want to "future-proof" any engine purchase. That's okay. Just don't look too far down to road that you loose sight of what is appropriate for the task at hand.

I'm sure you may be one of those talented ones that will be flying 3D in a few months (I know I'm not ), but that's not to say you'd want to get rid of a good Sunday fun flyer. Afterall, this is why you chose a larger semisymmetrical wing plane like the Hobbistar, isn't it?

My recommendation is to go modest on the engine on this trainer, then save up to buy that fire-breathing 3D barnburner when you're good and ready. My recommendation is a .61-.75 2stroke costing less than $100. There's a myriad of good engines in this segment: Thunder Tiger, Magnum, GMS, MDS, Tower Hobbies, etc.

Another thing to keep in mind is something none of us want to think about but unfortunately happen all too often - especially to beginners - CRASHING. If and when it does happen, I'd rather it be a $50 used 61FP than a brand new $250 MVVS .91.

To answer your question, the ST G75 and MDS .68 turn the same 13x6 MA about the same RPM. The MDS might even be a little stronger. I have a Pitts muffler for the G75 for use on another plane, so the MDS took its place and flies the Hobbistar just as well. Fires up on the first flip of the starter every Sunday morning - even with plenty of afterrun oil in the crankcase.