John,
...Looks nice in the "bones"...
Last AA,Sr. I flew had a left-hand shaft Fox 35, no tipweight...
The design doesn't need tons of power, really. Your alternate engines sound suitable. Small (I hope) point to consider:-- the Fox 35 works just fine with a uniflow tank 1" tall mounted ON the mount beams. That goes for upright and inverted installations. I haven't checked the FP or LA 25's to see whether they are sensitive to tank height relative to the engine...
Uniflow venting makes tank reference location just that much more critical that, even with muffler tapped pressure, you could notice a clear variance between upright and inverted run. ...IF the engine is sensitive to this. It appears you've closed-in the front end already, so I presume a 1" high uniflow tank is inside. I know a Fox 35 would be happy, and hope your alternatives will be.
I made up a break-in bench allowing simulation of the DIRECTIONS of vectors acting on the tank in a flying model. QUANTITY of the vectors can't be applied, of course, except in flight. The directions are enough to indicate response to a tank that is too high or too low -- riching where the tank is too high, and vice versa. This has succeeded in nailing tank height on about a dozen models, mostly using "odd" engines (many diesels, a sparker, other non-Fox35 pieces.)
It does not seem to have much to do with spraybar height above/below mount surface. It DOES have to do with the individual engine type... Oddest tested was the older big-case Fox 40BBSchn, mounted inverted. A 1" tall tank needed to be about 3/8" off the beams... Ran fine when the tank was centered for the engine, with spraybar anywhere from 1/2" to 7/8" "above" (i.e., towards cylinder head)the beam surfaces.
