On a boat that old you don't stand a chance matching gelcoat.. So here is what you do. Grind a small taper around the inside of the holes. Tape some sort of smooth flexible backing material (compatible with the resin. Styrene sheets will melt with polyester resin) with mold release agent applied over the outside of the cabin. Use whatever resin system you are comfortable with, Mix in a thickener like micro bubbles. Have ready cut pieces of glass handy and just build it up from the inside. When cured, move to the outside, take off the backing. Dewax, sand, apply marine filler. Sand, prime and
paint. You should be able to sand and fair it enough to not notice any holes. Gudgeon Brothers put out publications detailing the how to do this, can be done with gelcoat, but it will never match.
http://www.westsystem.com/ewmag/21/p..._practical.pdf