To start, drain the tank, wash it out (detergent/soap type washing), along with steam cleaning
, & pressure washing
. Which somewhere along the way you may wish to drill some drain holes in the bottom so that ALL of the petroleum products, as well as the oils suspended in the detergent cleanings, have an easy way out.
Then grind any areas in question back to bare glass, & assess the area of damage. Followed by repairs
with epoxy, & triax. Plus having a removable tank for that void, fabricated.
You really do want to go full on OCD when it comes to getting things clean, & free of contaminants. Perhaps even to the point of having someone test the laminate after you're "done" cleaning
. And prior to doing any glass work
Also, since it is leaking, that's a sign that there are cracks which extend fully through the laminate in some areas. So given how thick keel laminates tend to be, you're in for doing a lot of glass work
... that's fairly professional in quality. And one tricky bit will be when grinding in order to get your bevels for applying new glass, is not to remove so much material during this phase of the repair so that the internal ballast is less than adequately supported.
There's some great, albeit semi-scary, information & pics in the thread by Hooked & minaret when they repaired his keel stub/sump. It's several pages long, with lots of pics & descriptive captions, dealing with repairing serious structural laminates to a boat's keel mounting stub. Some/much of which will apply here.