I'm sure the C G problem is different with your choice of engine. I set it up with a Saito 100 which is lighter than the OS 91. Personally I think this plane would be nose heavy with the 51 as well. I chose 4 3/8" as my C G. For the 100 FS I had to put the rudder servo in the tail on the bottom of the fuse. I used a five cell 17000 milli amp battery back in the fuse against the servo, the engine sits right up to the fire wall (it actually touches it) with any excess of the front of the engine mount cut off and got away with one ounce of weight on the tail. This is with a 14 x 8 Master Airscrew, A wooden prop may be lighter. If you look at the wheel pants the ply mount is small. The pants crack where the block ends. You could remove it and put in a longer one or add to each end and laminate over it. I chose to fab a custom pair from balsa (block) and ply sides. The cowl is holding up since I isolated it with rubber grometts where the screws go thru. Good luck, Your going to love this plane.