FWIW......the older motors are usually simpler and more reliable than many of the "newer"motors.
Motor reliability is directly proportional to the maintenance it recieves
Have not stated what your motor is yet, what issues you are having. The only things we know are the boat is a 1977, the motor on it is either a 80 or 85hp.
If the motor is a 1977 vintage, it will be crank rated and not prop rated. This will be one reason it may not have the power you feel it should
Rating systems changed in the 80s
Keep in mind, depending on manufacture, the mounting pattern will have changed in the 80s as well
When was the last time the fuel system was serviced? Ignition system? Motor could be down a cylinder or two. Not because of age, however something simple.
If you and your dad do repower. Remember these:
Newer 4-stroke motors are much heavier than their 2-stroke counterparts.
Get motor AND controls
Motor mouning has changed
A 44 year old fiberglass boat may be carrying around a few hundred pounds of water below the deck. Make sure to check the transom and stringers
Thanks for the detailed response, a few things, the motor is a 1984 Mercury so I am not sure where it sits on the rating but it currently has enough power. My father is the one who wants less on it than it already has.
As for the older being more reliable if maintaned, I know they are solid engines and my vote was to have the engine rebuilt - already had a major service and tune up which helped a lot though I doubt it is down a cylinder because of how it responds and talking to the mechanic - however my father (the owner of the boat/enigne now) is determined it is old and needs replacing. Over the years growing up I never had much money so I always bought old boats/motors that were never in good shape and 4-5 times a year we would get stranded so that is what has driven his idea of old motors being unreliable. No changing his mind on this. The motor on it now left us stranded twice last year - we did 6 trips out in that boat between us - both times were human error ie I told him it needed a new fuel line because it had a hole in it so air got in, next was we ran out of gas because he decided to only bring about 2 gallons and go WOT... I normally go by 1/3 out 1/3 back and 1/3 spare but he thought he knew better than I did.
I know the 4 stroke are heavy compared to the old 2 strokes which is why I was hoping to find a newer 2 stroke from Mercury though I didn't see it on a local dealers site that I normally deal with so guessing they are not made anymore by Mercury and I have been advised to avoid the eTecs even though they are good motors - mainly because I am told repairs get expensive over time due to bad company practices though not sure how true that is.
As for if the boat is carrying water or not, I really don't think so, I rebuilt the transom in 2016-2017 along with checking for any water logged areas. No water was found in the deck or below deck but did find some small rot in the transom. It was replaced because a mechanic crashed it while moving it into the shop and broke the transom to where it was not safe to use anymore.
