Re: the shape of the old
rudder.
Would most likely go back to that shape if there wasn't any issue w/lift or the
rudder stalling in heavier air.
If you want to shape/more
lift could look
here for different shapes.
For more info on rudder design
here is a paper discussing how to make a better rudder/more
lift.
Re: the internal foam
Is the rest of the foam in the rudder wet? If the trailing edge was wet, would suspect the rest is also wet. If so would want to remove that wet foam also.
Typically one would cut a rectangular portion out of the side, dig out the foam and then only have a side patch to glass. Have seen many use a medium density 2 part pour in expandable foam as a replacement foam. This is the easiest way to
repair an existing rudder.
We used a
structural foam to make a whole new rudder for our
boat after a PO put on a different rudder w/no lift (a flat rudder). The new rudder works well, but IMO don't think you need to go that far to
repair your rudder. If you wanted to radically change your rudder design then it could be worthwhile to use structural foam to make a new rudder blade.
A final thought is if you had
salt water intrusion into the rudder
core, while you have it out and apart, you may want to double check the rudder shaft/internal welds holding the rudder stock extension plates/rods.