I had a split and cracked rudder some years back, being new to epoxy work I may have gone a little overboard however I am pleased with the results.
First like you I had drilled some drain holes and was going to let it dry out, however the more I looked at it and thought about it I decided "If I try to rebuild this thing and fail I will have a great excuse to buy that kick-up rudder I have been wanting." So I am going to ruin it or do it up right!
So where the crack/split was I ground out a very large Vee groove. I actually split the rudder as much as I could. I lay the thing in my garage for a week in the hottest part of the summer with a heat lamp shining on it until I was sure the thing was dry. It lost a lot of weight during this time, there is a lot of water in the things. It may be closed cell foam but it still holds a lot of water.
So with all the bad areas ground away all cracks, splits, swells, removed, also in the seam area where the long crack was, I removed a little of the foam filler as well, not much but enough to get some epoxy and glass globbed in there. Make sure to grind your bad spots out and then feather out to an area that is sound
Do some shopping online for epoxy products, the west system is way expensive and for where I live too hot (it cooks off fast) I also had some 6 oz biaxial cloth laying around from a boat I built, so I took some of that and chopped it up to fine small pieces, also take this time to measure out little FG cloth patches to fit in the groove, make several for each area ever larger until you have one that is going to fit all the way across and onto the good area that you feathered to.
Mix the chopped strands with your non-blush epoxy of choice and glob, stuff and cram it in the holes, cracks, splits. Let it harden till you can work it, Within 24 hours (as soon as it is hard enough to work with) grind it down flush (for the Holes) or a little low "Grooves splits big holes", clean everything real good and lay your previously cut and sized biaxial cloth patches in the groove, wet to get it to stick and continue building up with ever larger patches until you have the groove built up flush. At this point I then took a long piece of cloth and wrapped around the trailing edge to make sure it would never split again. Finally I vacuum bagged my rudder but you don't have to go to this extreme. It just saves so much sanding!
Once everything was cured I used an epoxy fairing compound (Micro-balloons and epoxy)to fair the holes and the edges of the glass cloth. Then I water proofed with a coat of clear epoxy and then a coat of white.
Just make sure to plan ahead so you do all your work at once. Don't let it cure too long in-between the epoxy glass work or else you won't get that super chemical bond, you will just have a glue bond.
Hope that helps and good luck!