Transom Capacity and Trolling/Backup Motor size Starcraft ss160

Not sure if this is the best place for my question..

I have a recently aquired 1985 starcraft ss160 with a 4 cylinder 50hp 1985 Merc outbooard. The boat is rated to 90HP max. It came with an anemic 4.5hp 1975 Kicker... Fishing in some pretty serious tides and waves and want my trolling motor to be an effective backup as well.

Seen and heard the guys out there with their small outboards revved way up to move their boat.. No thanks..

So my question is twofold.. How much additional weight can I / should I add to my transom.. I suppose I could try to enterpolate the difference in weight between a 50hp and a 90hp but that would only be a guess..

My first choice was around a 60lb 8hp yamaha, johnson , evinrude... add an extra 10lbs for a merc...

If I go up to 75 lbs then I can run a 15hp merc which should troll well and move me around effectively in a pinch..According to the NADA web site, the mid '90s mercs weigh about the same for the 8, 9.9 and 15hp

I am thinking the lower rpms to move the boat will keep me much happier fishing and probably not much consumption difference between an 8 and a 15hp motor.

My final option is even heavier with a 4 cycle 2 cylinder in the 8-10hp range but now the scales tip at 100+. Checked out a used 9.9 merc 4 cycle and could barely lift that sucker..

Just an opinion - I'm no expert. The transom is designed for a 90 HP 2-stroke outboard. That translates to about 300 lbs. Most transoms are always a bit over engineered. Assuming your transom is in good shape, you should be able to hang up to about 375 lbs. on it without overstraining anything, especially given that the weight is distributed between the main outboard and the kicker. Your current Merc 50 probably weighs about 200 lbs. So it seems you have a lot of latitude.