I would mount it sideways about 110 degree angel - then the muffler will be on the underside. I had my OS 91Fx mounted that way and have no trouble with starting or running the engine.

And the best of all very little cut out on the cowling, just for the engine head, throttle and the muffler.

I belive you will have a lot of mess on your left ving and left body side if you use the engine mounted upright.