I'm just installing camping equipment from an '83 Westy into my '85 GL. 
The water pump was also seized. I couldn't fix it, but found a 2.5 amp 
500gpm bilge pump at a hardware store ($30 ; VW quoted $60 for the 
original).  Being desperate, I bought it.  I realize it will probably 
turn my sink into a pressure washer, so I'm going to try to return it, or 
put in a rheostat switch to control the output (any suggestions on this?)
Later, I went to a marine supply store and found the same pump for $20, 
and smaller ones, too (150, and 300 gpm) for proportionally lower prices.
Otherwise you could try an RV store...

-MARK 
'85 Retrowesty