2wing
You might want to try purchasing another type of mount for that plane. I do not have any prior experience with DP aircraft, but I have encountered this problem in other kits. Seems a little odd that it comes out left thrust. I can understand the down. Most aerobatic bipes pitch to the top in knife.
Your idea of shimming would work too, as long as you maintained a strong mounting surface. The RC gas car people use over the counter shims, measured in the 1000,s of an inch in thickness, to set their clutches. These shims have about a 1/4" dia center hole. That was provided just in case washers wouldn't do the job.
Ernst Products makes 1, 2, and 3 deg thrust plates for changing the thrust angles of engines for flight trimming. They are mounted behind the engine mount. You might want to look at these also.
Silversurfer