Paul knows far more than I do about these things.
But just like heís suggested I have in the past cut out an old valve and glued a new one in place which was already mounted in a circle of Hypalon.
Itís a bit tricky keeping it all flat and not gluing the tubes sides together but perfectly achievable by a competent amateur.
If it's just the 'mushroom' you need to replace, there were some very good instructions on RIB Shops website about how to do it with the body of the valve still in place in the tube. I followed them myself and found them easy enough to carry out.
Unfortunately my internet access at work wonít allow me anywhere near the ribshop website to post a link.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club