If you are going to mount the 90SP in this, I would suggest cutting new versions of the parts that make up the engine mount box to extend it so when the SP is mounted, it is at the proper distance. I don't recall the exact amount I had to extend the box, but it was in the neighborhood of 1". If I were to do it again, I might be tempted to remove some of the right thrust and downthrust. It may make the plane tend to the left during takeoffs, so if you do change it, keep that in mind.
A bit late now, but one think I did was to hollow out the blocks that make the tips and bottom of the rudder and vert. stab. Try and keep the weight as light as possible. At the end, I only needed a 1 oz prop nut weight to balance, but I did pack the cowl with lots of stuff - 2 batteries, a ducted fan, and ecs and a copper elbow as an exhaust extention.

Scott