Just a 2 cent comment. Been flying for a bit now, and have flown with several different makes of engines, and sizes. 2 stroke and 4 stroke. I have to say, all of my engines start easy, idle well, transition from idle to wide open well, and run until I shut them off. A little attention to screws, rubber o-rings, fuel tanks, and engine mounts keep them that way.
If the airplane manufacturer calls for a .61 on that one, that would be the smallest I would go with. I think a .65 would be the largest. ASP is the best value you can buy, O.S. is great if you have the bucks for one. If you have really deep pockets, a Saito 4 stroke properly sized for the airplane would be great.
2-strokes will give you a bit more speed, 4-strokes will give you more thrust.
All of the above are simply my opinion, so, make your own choice. If it were mine, I'd use this K&b .65 I have on it.
A lot of engines can be bought second-hand in the classified section on RCU, and other discussion forums. They won't break your bank. Oh, by the way, don't forget about Evolution engines. Mine run great!!!
Just remember to have fun!!!