The Mid-star is an excellent plane. I built one and flew it for awhile before just recently selling it. I built mine stock but there were some mods that'd I'd put on one if I built another. In order of importance;

Must do things I had to do;
1. Put a small piece of triangle stock on top and bottom of the horizonal stabilizer to beef it up.
2. Put a Sullivan tail wheel on it.

Basic mods I wish I'd done.
1. Loose the wingtips.
2. Put dual aileron servos in it.

Wilder mods;
3. Take at least half of the dihedral out.
4. Cut the Rudder so that's it's vertical to the thrust line and double it's length.

If you really want to get wild;
5. Increase elevator about 1/2 inch.
6. Increase aileron by about 1/8 inch.

Mounting the engine sideways is also a good idea as the exhaust does tend to creep under the cockpit and into the servo area. I put an exhaust extension on mine and it helps but..

The Mid-star though flys fine without ANY mods. I could even knife edge mine with a Super Tiger .40 but it took a LOT of stick to hold it there.

Oh, also toss the Sig easy hinges and get some other brand (I ended up using Great Planes) as the ONLY easy hinges that ever broke on me where Sig's.