I used minicraft gelocat for patching holes when I had my 1988 v20 steplift.
https://www.minicraft.com/retail/wellcraft.htm
Glacier Grey was the hull color
Phantom Grey was the floor color
They have a gelcoat matching service . . .
I think the hull color was the same in 79, but I don't know for sure. Maybe they'll send you a sample batch.