Specification of DC motor
This motor is manufactured by Mabuchi. The part number 'RK-370SD-4045' actually tells you quite lot about it. Referring to the chart below, 'RK' means it has a round (circular) case and carbon brushes. 370 is Mabuchi's designation for motor size and number of armature poles. '4035' means 35 turns of 0.4mm diameter wire on each armature pole. 'SD' means it has anisotropic Ferrite magnets.    



Unfortunately Mabuchi only publishes specs for a small selection of their motors, not every variation. However if you have the specs for one particular wind (wire size and number of turns) it is possible to calculate the specs for other winds. Specs for the RK-370SD-2870 are:-

Voltage: range 4.5~9.6V, nominal 7.2V 

No-Load: 16500rpm, 0.34A (at 7.2V) 

At Stall: 36.3 mNm torque, 8.77A (at 7.2V) 

Your motor is identical except for having half as many turns (35 vs 70) of wire which has double the cross-sectional area (0.4mm2 vs 0.28mm2) and therefore 1/4 the resistance and 4 times higher stall current (at the same voltage). With half as many turns the torque constant would be halved, but with 4 times higher stall current the stall torque would be doubled. No-load rpm should be twice as high at the same voltage, as should no-load current (probably a bit more than double due to increased windage and eddy current loss at the higher rpm).  

However since apart from the windings both motors have identical construction, their maximum power and rpm ratings should be the same. For the same performance (rpm, torque, power and efficiency) your motor would need half the voltage. So we can estimate the specs of your motor as:-

Voltage: range 2.25~4.8V, nominal 3.6V

No-load: 16500rpm, 0.68A (at 3.6V)

Stall: 36.3mNm torque, 17.5A (at 3.6V)    

From these numbers we can derive the motor constants Kv (rpm/Volt), Kt (nM/A), and Rm (resistance) which are used to calculate performance. Mabuchi's performance graph for the RK-370SD-2870 is shown below. Your motor should perform similarly on 3.6V, but with double the current draw. I have marked the changes for your motor in red.  



Note that these calculations (including Mabuchi's own performance graph!) are only theoretical, based on a simple DC motor model. In practice there may be differences due to factors such as brush resistance, timing advance (may be forward, neutral, or reverse), applied voltage etc. To get the actual performance on a particular voltage you will have to test a real motor.