It is a bit of work but you can always cut the coaming and riser off with a hand saw. then draw a new larger cockpit opening.  Try getting in and out with the new size. When it feels right then you  build a new set of risers and coaming.  If you are out in rough water you will need a new larger spray skirt but those are available.  If you only paddle in quieter water you could even link two cockpits into a large one but would probably need to add some stiffners/ knees under the deck to maintain strength.  

OR  build a new kayak for yourself with a large cockpit and let them have the old one.  

Hope this is helpful.  I'm sure others will provide more ideas.

Ed