I often break in my RC Race car motors by running them from a single 1.5v D cell. If you can get the motor out of the car, it should run from one of these cells. If you can't get it out of the car, you should still be able to get it to run from one of these cells unless the gears are binding in which case you will have to take the car apart and fix whatever mechanical issues it has.
In cars that are left for a long time, the grease in the gearbox can turn hard in which case you will need to clean this out before getting the car to run.
As others have said, you will not get the car to run well on domestic batteries, ideally you want Nicad, Nimh or the most recent technology LIPO batteries. All of these require an investment in a new battery and charger so be sure the mechanics work well before investing in these.