I am ASSUMING that the rails are attached with wood screws now.?.? The rails should have been located over the plywood that is part of the roof sandwich. Anything screwed into fiberglass only is a failure waiting to happen. If they are in wood you could remove the stripped screws pack the holes with slivers (1/16" x 1/16" x 1/2" or so) of a hard wood (oak, maple, etc) and re-install the screws. Should hold. Epoxy is good as long as you are not planning on taking anything apart in the future. Can still be done but you may have to drill out broken screws.
The screws should have about 3/4" of bite into the roof. Longer might start coming through the inside My roof is 1" thick (a true one inch ... ie:16/16ths) at the center plywood strip.
If all the screws are really in wood today and the proper length and still 2/3rds of them are stripped then the bolt, washer, LOCK nut approach indeed might be needed.
Good Luck