Pantera, ask your machine shop what they would charge to put in larger valves. You could take my bare castings and likely have about the same money into a brand new set of heads vs. buying a used set or a set that doesn't fit your app.

Small stuff, 21's and 23's, may have a 25ish 4-blade offshore.

Maybe try a 23 see what happens...
Head thing driving me so crazy ready to do anything just to get motor back and go for a ride.