What epoxy did you use on it? Any special surface prep?

I will do it this winter. I'll sand it and use regular Raka epoxy with a few layers of cloth inside and out.
The damage is like a 8 inch tear. So, I'll just clamp the tear back and epoxy it shut and then sand the outside and put a few layers of glass over it and do the same to the inside. I will coarse sand it and epoxy will adhere fine.

That sounds like a good plan. I've heard talk about using G-Flex epoxy on Royalex, and for straight bonding (not glass laminating) apparently methacrylate adhesives (Plastic Welder by Permatex or Devcon) work really well. So, that's why I asked.

I remember having a lot of trouble bonding tie-down loops (for thigh straps) to a Royalex canoe many years ago, but the toolkit of adhesives has improved since then.

And yes, it's so easy to do repairs when both sides are easily accessible (as in a canoe). A couple of days ago, somebody was discussing online a 'stiffen the deck' repair inside the day compartment of a Zegul kayak. Not a fun job.