This is what I would try as to removing the existing bolts.Try turning the head of the bolt with an electric drill as fast as you can, and pull up on the broken part of the cleat. This will jam the nut to the underside and with some friction and the bolt turning fast enough, it might come out. If you could use air tool and it was a hex head bolt that would work, but I am sure you are dealing with a slotted flat head bolt. That will be harder to spin. If that does not work then I would take a sawzall with metal blade or dremel and cut through the cleat and bolt. You may not be able to retrieve the stud or nut, but what harm can they do?
Now to fix without cutting access....Blind Rivet Nut will do the trick. Counter sinking holes deep enough so the head of the blind rivet will sit flush allowing the new cleat to sit down flush. Be careful not to over counter sink and weaken the hole or larger counter sink than the base of the cleat. Also, countersinking holes in fiberglass is know to help prevent cracking. See Blind nut rivet info here... Blind Rivet Nuts - Bolt Products, Inc.
I see SS 304 and 316 listed here... http://www.sherex.com/pdf/Dejond%20T...%20Catalog.pdf
And don't forger to bed the new cleat.