The standard o ring for the tube to water pump connection is 25* 3. these dimensions are metric. If you have wear in one part you could try a 1 inch * 1/8 dia o ring. The 1/8 o ring is very slightly larger in diameter and the o ring is is slightly longer (25.4 mm) so you are stuffing a little bit more rubber in the hole which may make the difference.
I have reguarly made o rings using o ring cord, a sharp knife and instant glue. Got me out of a hole a few times at work but really only works best on larger diameters. On smaller diameters it is hard to get an aligned joint in the o ring.
I am not suggesting that you tear down your non leaking bike to try this out though.
All opinions are given with no warrenty, expressed or implied!!