Well where I'm living we are at 4500 ft. above Sea Level, so there is a quite a loss of engine power, about 25% or so, in the Summer months due to the really high density altitude with warmer air, so over powering a model at Sea Level is not overpowering one here.

That said, I firmly believe in throttle management and would rather overpower than be underpowered. You could also run the engine a little rich, or restrict the throttle travel if you don't think you can resist using what's available. I think that any of the newer 50cc class engines would be a good choice for you. By "newer" I mean those designed for model aircraft use vs. converted weed eater engines. As noted above, any of those engines will allow for a much larger selection of models when you decide to get a new one.

I don't want to get into the "Chevy vs. Ford vs. Chrysler" type debate on brands since I believe they're all good engines with a few pluses and minuses to each one, but I run ZDZ...