Yes it is worth the work to stop dragging the old saildrive leg around. It also removes a serious flooding risk—if the saildrive seal ever lets go, you can sink your boat pretty quickly
. Closing it off should be a priority.
Basically, you're just patching a hole in the hull.
Wash the hull with a good dewaxer/degreaser like Interlux Fiberglass Prep Wash 202. The reason for washing the hull is to prevent the grinder from embedding grease/wax into the fiberglass when you grind it.
Grind the edges of the hole to a 12:1 taper based on the thickness of the hull there on both the inside and the outside if you have good access. If the hull is 3/4" thick, you'd grind a taper 9" out from the saildrive hole.
Seal the hole off temporarily with a piece of thin wood, plexiglass or HDPE coated in release compound on the inside or outside, depending on which side you want to repair first. I'd seal off the outside and start with patching the inside, since gravity will be on your side then...
The seal can be duct taped to the hull and should follow the hull's curvature fairly well.
Coat the tapered area with epoxy and then lay up fiberglass patches. The patches should be differ about 2.5-4" in diameter and the smallest should be about the size of the original hole, the largest about the size of the tapered area.
There are two schools of thought on applying the patches. The first is that the smallest should go on first, and that you build up the hull patch thickness with increasingly larger patches....the other is that the largest goes on first and you build up the thickness with smaller patches. Either will work, but I prefer the former method as I think it makes a stronger repair. You will need to use a cloth heavy enough to make up the hull's thickness. If the hull is fairly thick, you'll probably want to use more patches that are closer in diameter to each other. Try to use enough epoxy to thoroughly wet out the fiberglass but no more than that. Excess epoxy contributes weight but little strength to a repair.
If the hull is very thick, try not to build up the thickness more than about a 1/4" at a time or the epoxy curing can generate enough heat to cause problems. I'd recommend using a no blush epoxy like MAS epoxy. Once the patch has cured to the tacky to the touch stage, you can add additional patches. If it fully cures, and you're using West Systems, you'll have to rinse and scrub it to remove any possible amine blush.
Once the fiberglass cures, remove the board sealing the hole on the bottom of the boat and wash the area with the Prep Wash 202 to remove any release compound. Sand the exposed patch lightly and coat with epoxy and glass over the hole from the bottom of the boat.
Once the patch cures completely, coat it with Interlux 2000E barrier coat, as a primer and then apply bottom paint to the outside, coat the interior side of the patch with Interlux 2000E.