Hi esimonou, 

yes.  on a guillemot strip build this can be a very straight-forward project.

as we get older, we tend to get a bit bigger if not a bit less flexible too.   a bigger cockpit can really help.

if you built the guillemot according to plans, the cockpit opening is actually relatively short in length (29 inches) compared to some of the more modern designs like the petrel (31 inches).  i am not very flexible and i build my cockpits to 31 3/4 inches.   so its very reasonable to suggest doing what you are suggesting....and now you just have to work out the details.

in a nutshell...

• come up with a pattern for the new cockpit
• cut the old cockpit out and enlarge to whatever your pattern is.
• build out your new cockpit just like the old one (in cedar) 
  • apron (if that is part of your plan)
  • risers
  • coaming
• sand it
• glass it all up inside and out
• sand it
• varnish

depending on your modification (if you are just adding length or do you need width too), you may need to reposition cheek plates.

the back of the cockpit should not move...so when we talk about enlarging a cockpit, keep the location of the seat the same.

fwiw, i would begin with a sketch of what you want to do and plan it out....becuase, while not overly difficult, once you start cutting, you are committed.  

in terms of the work, i would do it with a hand saw like the beading saw https://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/booth-tools/beading-saw-with-woodpecker-tooth.html 

or a bonsai saw https://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/booth-tools/bonsai-saw.html

i would favor the bonsai saw as the curves get tighter but i have done this kind of work with both of these tools.

feel free to post you sketch and your ideas and happy to help to bring another set of eyes to your plan.

h

   I've had the same thought about my 20 year old Chesapeake 17 - osteoarthritis in both knees combined with long legs (36" inseam) and old age (74) have combined to make getting in and out a bit inconvenient. My solution for now is to build a Mill Creek 16.5 with its 8 foot cockpit - but I still think I will modify the cockpit on the Chesapeake. I've never been able to get in butt first even when I was 20 years younger but the "lift" to get in and out is getting more difficult - increasing gravity I suppose (what else could it possibly be?).Will be interested in seeing how your project comes out. The Chesapeake has a deck beam at the forward edge of the cockpit so that will be a consideration. I don't know if that will be a problem on the  Guillemot.

   My knees like a less "keyhole" cockpit.  In the Cheasapeake I cut the thigh braces off a long time ago.