I have one w/ a TT .46 and it's got plenty of power...I can (just barely ) fit a 12x4 APC which IMO is a perfect prop for this combination.

I think the ply/glass engine pod/mount would be taxed to the max w/ a big 4 stroke like an .80 and especially a .91. (not to mention the prop clearance issue ) And I think with that much power the tail would rip right off unless you brace the heck out of it...
My entire engine pod vibrates and shakes quite a bit horizontally at certain throttle settings. A .70-.72 4 stroke is about as big as I'd go, and as big as I've heard about. The .46 will be plenty. Mine weighs about 6.5 lbs. and flys great

I just completed a Seamaster clone w/ a Tower/Aquacraft .46 in it and it's surprisingly fast with it, even w/ a 12x4...