That looks a lot like a Starcraft Holiday with a replaced windshield. If 16ft, it's rated in the 85-90 range.
We have a 1971 SC SS 16, have a '76 Merc 850 on it, a great match for the boat.
I believe our '76 is the 1st year the 850's had the elec ignition, I wouldn't go older.
The 80's 75, as mentioned, is the same motor prop rated, and it has the newer T&T, and plus.
Last year I sold a good compression and running '84/5 115 for $500. (wish I kept it now)
This fall I bought a project boat to get the motor, a early 80's Mariner 90, for $600 - I over paid (long story).
I'll find out come spring whether I win or lose on it.
With that rotted transom, go over to the Starcraft rebuild page, lots of help there.
Dan