Please post some pictures of the old hull repair. Be good to see what you're seeing.
Concerning the floor, one way you can address the soft spot for a very temporary fix (cheap, quick & dirty) is to remove just the deck skin, which is about 1/8" thick, around the perimeter of the soft spot. This can be done with a Dremel tool with a cutoff wheel or better yet with a Milwaukee grinder and cut-off wheel. Lift off the skin fiberglass and dig out the rotted plywood down to the backer glass layer under the ply. Think of it like a sandwich; the stack-up is: the glass you stand on, below that is the plywood core and below that is a fiberglass backer layer. What you don't want to do is cut through the backer layer of fiberglass. When you've removed the bad plywood in the area you have exposed, cut a new piece of plywood to fit in the area you have prepared. Measure the thickness you'll need... it will likely be 5/8", but measure anyway. For a dry fit, it should be about 1/8" below the surrounding deck level.
After you have created this new plywood insert (called a dutchman), mix up some polyester resin and paint the dutchman with the resin to seal it up. After the resin has has hardened, the next step is to bed the dutchman into the prepared deck cavity with thickened resin. To do this, mix up more resin and thicken it with Cabosil to the consistency of store-brand ketchup. Cabosil is a thixotropic thickening powder which will thicken up resin depending on how much you add. After the thickened resin is prepared, butter some into the deck cavity about 1/4" thick. Next, "wiggle in" the dutchman so that it settles into the cavity and its surface is about 1/8" below the level of the surrounding un-disturbed deck. Let that cure to hard then set in the piece of removed deck skin with more thickened resin. Likely the level of the patch will not perfectly match the rest of the deck but that can be addressed by either sanding the area down or by fairing the area with resin thickened with micro-balloons and Cabosil.
Again, this is a very temporary fix. If you have one soft spot in the deck, the rest of the deck will go soft sooner or later because water's gotten in there. The above fix will will be visible and is not a substitute for a full "deck job". It will, however, get you used to working with resin and give you a bit of a sense of how these boats were built.