If you can lift the access port up can you epoxy the nuts in and then install the bolts?
Otherwise fiberglassing the hole is not hard, as someone above said. I would make the repair patch and hole oval so you can put the patch inside and then build up the outside.
You can see what I did to my boat if you go to this link:
http://owners.aquarius-sail.com/phpBB2/viewtopic.php?t=328I made several holes to get access inside.
