MacGregorSailors.com

Surv69, I understand now that you have removed the inner liner. In this case I would do the following from inside.
Make a pattern of the area to be reinforced. Transfer the pattern to either 1/8 plywood or 1/16ths or 1/8th inch fiberglass panels that you can find at RV shops. Make props of 1x2s of various lengths to hold up the panel. Sand down any major roughness of the deck underside. Clean the underside of the deck with acetone or alcohol as best you can. Wet down the top side of your panel with a slow set time epoxy, place it up against the underside of the deck. Jam your props into place, insuring a tight fit every foot or so. After the panel cures for 24 hours, remove props and repeat until you have as much thickness as you desire, say 3/8ths inch. There should be very little mess on you or your interior if you spread a painter's drop cloth or plastic film to work upon.

Btw, my screen name is Night Sailor because it is often too hot to sail on TX lakes during the summer in daylight, so we set sail after sunset and go until the early morning hours, sometimes all night if it's a weekender. When I was at a dock full time, some of my dock neighbors thought we never took our boat out because they never saw us on weekend days when they did their sailing. They only sailed daylight hours and tried to be back in the berth by sunset. They had to contend wth heat enough to blister your skin on anything metal, plus jet ski, water ski and 100mph penis boat drunks, and constant wakes.
They missed the moonlight, smoother water, night birds, navigation by stars, much cooler temps.
I find sailing at night increases one's sailing skills because you have to rely more on your sense of sound, feel, smell, and hearing. And on the darkest of nights, it can be a substitute and practice for navigation in unmarked channels as if you were in a fog bank.

My 1975 Venture has the same problem, the deck around the mast is soft. I drilled a hole in the deck to install some quick release connectors for the light and VHF antenna and noticed the wood core is fairly rotten. I was wondering what you ended up doing to reinforce your deck?

Back on the subject of overhead fibreglassing, I find that plastic 'cling wrap' works really well for any small repair, especially overhead.

Lay the cling wrap on a flat surface making sure it doesn't crinkle. tape if required. Lay glass and resin as required on cling wrap. Then place the exposed side of the fibreglass to what ever needs glassing. This way you can minimise the amount of resin in the glass and it stop the drips on you and everything else. It tends to stick quite well, but you may have to hold (or check on it) until the resin sets. This works especially well for any small jobs, say 2"x2". When the glass sets, simply peel off the cling wrap. Because the cling wrap is clear you can easily see what you are glassing.

My other trick is to use a diamond tipped grinding disk (on an angle grider) to prep for and profile new glass (including removal of sharp bits which cut sand paper. The disks are generally sold for 'dry cutting' bricks etc and cost about $10 for two.

I could not resist coming in on the use of filler. Micro beads are fine but cost....In largish use applications, I use non scented talcum powder...a whole lot cheaper and just as good for strength.
Once moulded a pointy bow on a 23ft. 'Tankard'. Then had the misfortune to run bow first into another yacht. Did the moulding collapse...No!. Punched a hole in the other boat...And all that moulding was fibreglass resin/ hardener and TALC. to stop drips in an overhead application, just add enough talc until it can be plastered on like sticky goo. It will not drip but WILL stick. Once the moulding is shaped, dust with more talc and then as it slowly sets, you can knead it and shape it up a bit better with your hands...just like preparing a loaf of bread with flour.
puggsy.