My experience with fiberglass runabouts of the day were that they were light, maybe 500# for a 14' runabout, closed bow, one piece windshield, 2 seats, maybe a splash well, maybe not and a floor, made with a matt or two of woven fiberglass roving, some resin, a couple of wood stringers, a wood transom core, a strip of wood across the inside of the dash and that's it.
The 77 Merc 50 cross flow was in production for many 10's of years and was as good an engine as could be bought at the time. I had an 85 Merc of the era.
50 hp on that boat, if like I described would be a nice combo and upper 30's would be expected. FIL had a boat like I described and with 2 people and gear powered with a rude 40 would run around 30 +. 50 on that boat would have been just right.
Before you set out, get in the Merc archives and look up what you need to do to the engine before you leave. If this is your first boat look at the bottom of this posting and take a USCG boating safety course. You won't regret it.