The motor I messed with pulled 4 amps with a pair of vise grips loosely clamped over the shaft to give it some load while turning
Not a real accurate test but I would guess with properly figured pulley sizes to get the right rpms at the water pump a normal motor would pull 4-5 amps at most. Just as a rough guess I would say it can handle about 100W of power without burning up so 5 amps would be a safe load on the motor to keep it lasting a long time. At least this was with the motor I used. I don't think they are standard sized and there is probably a huge range of sizes of these things. The wiper motor on my Astro looks about twice as big as the one I tested so it can probably handle a 10 amp continuous load without any effort.
Really though without the thermostat there won't be much backpressure for the water pump to fight against so I really can't see the motor needing more than 2-3 amps to actually pump enough water to keep a 4 cyl cool.
On a side note it takes a bit more rpms than a cordless drill has to keep a big v8 cool. When I was putting my kit car together I didn't have a belt for it so I just chucked a drill on the water pump shaft and duct taped it to a board and locked it on full speed. The car would overheat slowly but it let me run it and get the bugs out of some of the other engine stuff. So when trying to estimate the pulley sizes I would say keep them close to 1:1 and that should give a good enough speed to the water pump.