How do mend an air valve on an inflatable kayak?

I have an Sevylor sit on top inflatable kayak. It has two chambers, each has it's own air valve. If you are familiar with inflatables, you may also be familiar with the name"Boston Air valve". Well I don't have that one, I have the regular plastic one that has the one way flap inside the stem to prevent air from escaping once inflated, and then you insert the plug and push it into itself. I have been told that it is not repairable. And of course there is not a replacement bladder for this kayak. I have tried every kind of sealer, adhesive, tape, you name it , Ive tried it. The main problem is the bottom of the air stem has cracked, probably due to exposure and everyday use. I was hoping to adapt a Boston Valve in it place. Any ideas?