I have powered mine with an old magnum 40 2 stroke with a pitts muffler, had a very scale sound with plenty of power. It had so much power that at idle it would float all the way down the runway. It then got the Magnum 30fs. Very scale powerplant and would idle slow enough to land. It now has the Saito 40. I havn't had balance problems with the firewall in the stock location and the prop ends up where it should. It is a little tight in the back of the engine though. As stated it is a builders kit. It isn't hard if you know how to build in the older style. No laser cutting or ply box fuses. I learned to fly RC on mine (not a good idea) and it proved to be a sturdy little airplane. Mine has seen its share of the ground. I have 2 threads out on it, I recently converted it into a pa-18, also I made an clipped wing for it.
This is a much better 1/6 scale cub than any of the other ones out there. It is lighter, more scale, better looking and better flying than the Goldberg or Great Planes. The only strike against it is that it doesn't come with dummy cylinders.
Here links to the threads I have on it
http://www.rcuniverse.com/forum/m_25...tm.htm#2578593
http://www.rcuniverse.com/forum/m_25...tm.htm#2586828
Ben