I've been looking into adding hobby electric motors to cars, and realized that a really easy way to do this could just be to use an RC outrunner.
For example:
HL W60 35/ 200KV 6S Outrunner Brushless Motor for RC Multicopter-in Parts & Accessories from Toys & Hobbies on Aliexpress.com | Alibaba Group
That thing is designed to run at 22.2V (6 lithium polymer cells), 27A stall current. At an automotive 14V, with no load it would be turning at 2800rpm or so but I doubt it has enough power for that. Back of the napkin calculation says if the pump has enough resistance to make it turn at 1400rpm, it would be drawing around 8.5 amps, perhaps 120 watts demanded from the battery/alternator.
You could drill a few holes into a metal disc to bolt it to the pump that gets rid of the pulley, and it would stick out only 2 inches from where the original pulley was. Since the diameter of that motor is smaller than the diameter of the pump, you could easily make a bracket with 2 pieces of metal.
Add a total overkill brushless motor controller for 20 bucks:
Turnigy AE-45A Brushless ESC
Running it at just 10V default (with switch to full power under throttle) would probably reduce power draw to 40-50W, which is fantastic.