James,
First, welcome to RCU. It's a great place and you should get a lot of terrific info.

I agree that the OS would be a bit much for that plane, and would also agree with a Norvel. They make an 0.061 with throttle control.

Here's their catalog page for engines in your size range: http://www.shopatron.com/index/100.0.1979.1981.0.0.0

Keep in mind that the Herr Cub is relatively small, so you'll want to keep it as light as possible. Adding throttle will add weight, but if you're careful, and use something like the GWS Pico servos, receiver, and maybe Lithium Polymer Battery pack for the on board stuff, it should be a nice, light, good flying airframe.

Good luck with your project,
Dennis-