The autostarting generators I've seen have all been giant hospital-sized generators or some such, starting at $8000 or so.
One idea I wonder if you might like would involve using one or more off-grid type deep cycle batteries and an inverter, because you're right, air pumps don't take much power to run. I think you could keep your battery on a trickle charger to keep it charged, and have a relay in the line that switched it on whenever grid power went down. I know that's not what you asked for, but this might make a pretty inexpensive and reliable solution. If you went too long without power, you could always recharge your batteries with a running generator (or even your car), and then it wouldn't have to be automatic start.
