I just did my Viking 33 (C&C by Ontario Yachts using the same "grey gasketing" used by C&C and CS). We cut new 1/4" Plexiglass and simply bedded with clear exterior silicone caulking and pushed in the gasketing...the secret to this is to pull up with your fingers into the frames until you feel it gripping the plexi. Put the gasketing in hot water first...it makes it very pliable...and then wipe it dry.
Prior to this, of course, you have to be fastidious about getting all dead gasketing/butyl tape/goo out of the frames, and you also have to mask off the plexi, save for the 1/2" that's going to be inside the frame. You should also mask off the window hole to keep silicon from dripping.
The gasketing is available from Holland Marine Products
in Toronto for $2.95/foot...not cheap, but I think I used 14 or 15 feet, and the plastic was about $20, so $65 and two hours of work for sparkling new portlights. No leaks, even when I powerwash them.