Hello George,
The 'gelcoat under glass' suggests someone did a poor glass repair job. Gelcoat should be ground off clean before glass is applied.
Having seen the rudder construction diagrams from the manual, I wonder if one could just cutout a large panel of glass from the 'wet' side, replace any internal wet foam, then glass over it again (properly)?
Could be done with the rudder still hanging. On a couple sawhorses even better. If molds are available and/or shell halves easily made even better.
my $0.02