here are relevant links :

http://www.whalepumps.com/

http://www.bosworth.thomasregister.com/olc/bosworth/home.htm

http://www.bosworth.thomasregister.com/olc/bosworth/page3.htm

I haven't checked them for a while - dunno if they still exist.

I have the guzzler 500 , it's HUGE, its intake is 2" diam'.

If you plan on using 2 pumps, think about the guzzler 450 .
they are 1.5" diam & few builders here testified it's enough for them.
they are good strong pumps.
I assume your kayak is not take-a-part, so you could install them
flushed with the bulkhead.

a word about your venture: the materials you use should survive use in salt water. I'm not sure if a valve from a dust mask can do it.

good luck,

Erez