Nick,
I have a photocopy of an 85 250 parts manual courtesy of Denis Myers that is the same in the motor but differs in the radiator shrouds and the linkage set up. If you can't find an 84 copy you cane get the 85 one from me. I had the same problem with the crank/water pump seal. I don't think Devcon likes the coolant as it goes mushy after a year or two. I will have to check mine before i go riding again to see what it is like. Perhaps you can Devcon the seal ring up and machine it out and live with the enevitable rot. Have you got the seal for it as they can be hard to get.
Cheers Bill