A 91 four stroke is what you want. If you mount it inverted you can hide most of it but you'd be well advised to put an on-board glow lighter for flame-out insurance. 4 strokes are more reliable but they still quit. The model comes out quite tail heavy. My first suggestion to you would be to redesign all the tail surfaces. Stick and sheet would be substantially lighter than the solid surfaces indicated. Also, plan on putting your rudder/elev. servos under where the pilot would sit. Adding weight to an airplane to balance it is a sin...I had to put a Harry Higley Heavy Hub on mine to get it balanced, without said servos installed. I'll send you some drawings if you'd like.

Thought I'd attach some pics of JSM Birddog finished.