How will a motor respond as a generator?
You need to know your motor's efficiency, or its winding resistance, or its back EMF at 1000RPM. Winding resistance is probably easiest to measure so I'll start there.
Best is to find all of these in the motor's datasheet and verify that they are (approximately) consistent as below : if they aren't, that indicates roughly how much you can trust the datasheet.
If its winding resistance was 2 ohms, at 1.2A it would drop 2.4V, corresponding to a back EMF of 9.6V at 1000 rpm (or, 104rpm/V). Wasted power would be 3.6W (or 25% of 12*1.2 = 14.4W) or 75% efficiency (slightly on the high side for such a small DC motor).
So you could expect 9.6V into no load, or 7.2V generating 1.2A (8.6W) or at 1000 rpm.
Running it as a generator at 2000rpm this would correspond to 19.2V into no load, or 16.8V generating 1.2A.
This should let you work out likely performance once you know the characteristics of your motor.