Yeah, I agree that it seems like it needs beefing up.
is certainly pretty tough, I think fiberglass
tensile strength is like 20,000PSI even for just-ok layups, but I don't really know how to interpret that, maybe someone who does can chime in.
It seems like a lot of strength, for example if your backing plate is 6"x6", thats 6" x 0.25" x 4 sides x 20ksi = 120,000 pounds to rip that plate through along it's border.
But that's not how it works, the load isn't perfectly distributed along that border at all times, and once stuff is flexing and moving, tiny problems start, it can move more, it breaks more.... I end up going with my gut on this stuff, and it seems like this "quick workaround" for your busted inboard is turning into a structural engineering project
It seems like something spanning the length of the transom is a good idea. But doing this inside is going to suck, in that cramped space, dealing with making a fiberglass layup
lying on your back looking up at the inside of the transom.
Maybe a couple 1x2s, stacked one on top of the other, and glued in place - they'll individually be easy enough to flex to fit the shape, but when they are glued together they'll be nice and stiff. Put two horizontal sets of them, one where your top engine
mount bolts go, one where your bottom engine
mount bolts go. That way you are at least done in a day.
Me, I'd just focus on the inboard...