For a .25 size 2-stroke, take a good look at the OS .25FX. It's one of the most reliable .25s out there.

I have a lot of hours running the Magnum .15, .25, and .28. They are good engines, but the carbs are really finiky, and the .28 vibrates a lot. If you really want a Magnum, take a look at my "magnum engine tips" at www.midatlanticcombat.com. The Magnum carbs have problems leaking air due to problems with the O-ring seals. Magnum is in the process of phasing in some new engine designs with new carbs that might be better, but I don't think it's reached the .28 or .25 yet. (the new .15 is due in Jan I think).

The OS LA .15 will give you "scale power", which will feel rather under-powered for a model. It's a fine engine over all, but not a great choice for that plane. Ditto the Mag .15 btw.

I agree with the above posters that a .30 4-banger would be perfect for t hat cub. A .25 2-stroke with a pitts muffler and a big prop wouldn't be bad either. I've found that a larger prop and the slimline pitts muffler changes the sound quite a bit from the "normal" 2-stroke whine.