PLJ;
As for mounting the Saito at 90 degrees there is only one problem. Centering the engine for the Cowl, to allow the exhaust to deflect properly. As you've noticed your tank location on the senior is fairly high. Your Thrust line is fairly high as well. Having enough room for the exhaust to deflect outside the cowl won't be a problem. But removing the cowl for maintenance may be! Without butchering the cowl to much. For Balance purposes the engine will have to be a little more forward than recommended. Balance point set at 4" or right on the spar works just fine and there is no weight added/Battery under the Tank shelf. With the Saito 72 the Senior will jump off the ground, so be ready for it.

If and when I ever do another one even one of the Senior kit's on the shelf. I'd probably go with a split cowl, as this will allow maintenance to the engine with ease. Then an inverted FourStroke could be installed with only the needle and exhaust protruding. As I've done on 1/4 scale CAPS (90 deg enclosed)and (Inverted) engine for Mustangs as on many others. I've always hated even cutting a small hole in a good looking cowl, so I look for alternatives to not cutting it up.

Hope this helps