Most Glastron's have hull drainage problems, poke around under splash well.
Also any holes drilled into transom might have leaked over time, and wood rot set in.
Never saw one apart, but there's a guy on Facebook that restored one, and I think Murray (Hydronut) has one.
Looks like you have a good motor, late 80's early 90's 3 cylinder Merc/Mariner's were good motors.
How's it run ?
I used cheap Home Depot indoor/outdoor carpet, looks OK.
Dave used marine carpet in his V-153, went around seat and could be taken out to dry or clean, Looked VERY nice !
Challenger / Merc 115 HP ... Very cool !