It's mostly basic carpentry with some resin.. Don't be intimidated.
I've noticed people(and myself in the past) will go to great lengths to avoid re-coring.
So ask yourself honestly which is more difficult?
1. Drilling 20 holes, injecting resin into them, getting resin everywhere, and in the end having it fail because resin is hard but brittle. There's a reason a coring is needed.
2. Buy one of these.
Best case scenario. cut the outer skin, remove we rotted core(probably just come right out) Do you best to level the remaining core(if any), Cut wood and lay in the wood with resin, more resin on top and reinstall the top skin with some weight on it. Half day project
Worst case. The outer skin is a mess or wont come out in one piece and the rotted core isn't that rotted so its hanging on for dear life. You could work
your butt off trying to remove it but i'd buy some self leveling seacast ahead of time(i'd use this over ply anyway). Remove wet core, pour in self leveling seacast, have a beer
while it cures, cover with a few layers of fiberglass
, sand(itchy part) and then paint
. Its not rocket science.