[Little]

I don't think the water pump for a 903 is very complex. Shaft, bearing, impeller and seal but I've never had one apart that I can recall. Rebuilt ones were cheap so most were just swapped out.

If you keep the water level above the heads and don't work engine hard enough to get the heads hot you should be ok.

if you lose 1 gal/ 20 miles a 5 gal pail should get you home. Our 86 had a block heater leaking coming home from Quartzite and we rigged up a hose to the filler. Wife turned on the water pump for maybe a min once in awhile and we made it home without issue. We were about 80 mies out. Good luck.