I think a TruTurn or a Dave Brown about 1-3/4" would look great. Like Kingwood B said, it might have more of a gap between the backplate and cowl, but of course that depends on how much you had to start with. My question would be do you have enough ground clearance for the 14" prop? The .91 will turn it with no problem, I turn a 14-6 with a .65 Saito to about 9700 rpm on a different plane, but it's a tail dragger. That engine and prop combo on a Senior should be able to drag it and a couple of others around the field. The Senior isn't gonna fly that fast, might want to crank the pitch back to a -6 or -5, but you might be able to over-rev the engine. Just curious, but why a .91? That would fly two Seniors. A guy at our field put an .82 on his, and has to fly it at 1/4 throttle. Make a helluva towplane though.

Seniors make damn good tail draggers BTW, if you can revise the tail to include tailwheel steering. Mount the mains with the front of the gear leg (aluminum type) about an inch forward of the L.E. of the wing