i am assuming that you already have your 17LT built by the way you wrote the note.

without speaking to the aesethetics of it, i don't think you need to worry about the strength of the boat/sides holding up if you cut an oversized cockpit the same size (approximately 50 inches long) as a wood duck as long as you don't try to make the cockpit so wide that you would need to cut into your sheer clamps.

i would approach it by

1)  first creating a pattern for your new cockpit (coaming risers and coaming)....and confirm fit/dimensions.

2) remove the old coaming by cutting it off the deck (i would cut it parrallel to the deck so you at cutting only the old coaming off....not the underlying deck

3) cut the new cockpit hole with plenty of spare wood to the inside of the pattern.

4) build out your new cockpit - risers first then the coaming itself similar to how you built it for the boat originally.

you will probably end up cutting through the existing deck beam becuase it is immediately in front of the front of the normal 17LT cockpit.  i don't this this is an issue.  it is there primarily to provide a form to bend the deck over and may provide a bit of additional support if somebody presses down on the forward part of the coaming.  but many designs use a temporary deck beam as a form and once the coaming is built, remove the deck beam.    depending where your new cockpit front is, you could craft a new deck beam to fit in that location if you thought the support was critical.  you may, however, be so close to the front bulkhead that there is no reason to even think about it.

as a final check before starting, i might get the line drawing for the 17lt and draw in the modification and e-mail a proposed design to CLC.  i suspect they would warn you off if they thought you were doing anything that was structurally not workable.