If you're buying a new engine, the Tower 75 or the GMS 76 or even the ST G90 are all excellent products, more than enough power, priced reasonably, easy to set up, break in, tune and fly. ThunderTiger & Magnum also work well, although TT service in my experience has been sub-par. My personal feeling is that OS is over-priced, and that they have been coasting on their reputation for the last few years. 60 size engines for a new purchase are more or less obsolete, since the 75's are the same crankcase bored out for more power with less weight. Now is a good a time to learn that you don't zoom around at full throttle all the time anyway. Use half to 2/3 throttle for level flight, full throttle for vertical climbing, and throttle back to idle whenever you have the nose pointed at the ground.

If you don't mind the added complexity--and cost, I think the Saito 91 is perfect for this model--A) because it's light (always compare 4-stroke with light muffler to 2-stroke with heavy muffler); B) it has great low-end torque, well suited for this type of model; C) it runs inverted no problem; D) I love the chug-chug sound--even if it has nothing to do with flight performance, how it sounds does matter to some folks. But I like a smooth running 2-stroke just as well, and they are less complicated. No valve adjustments to fool around with. Most of my 4-strokes don't require a lot of attention, but a few do.

I have HB and HP engines, like them well enough, but don't think I'd buy anything new that is not currently popular, mainly because it helps so much to swap tips and seek advice here on RCU.