What is the actual stall current of a Mabuchi RS550VC-8518 12v DC motor (out of a Dewalt 12v battery-drill)?
If you are searching a replacement motor, then you should firstly check if the dimensions match.

The second importan thing is that you may have already discovered is that the same size motor can have different specification, how come? It's because of the different windings, that motor is built for. 

Same size means almost same nominal power, but different nominal working points are possible. 


Tiny wire, lots of turns: high voltage, low torque (amps), high revs
Thick wire, small number of turns: low voltage, high torque (amps), low revs  


How you'll find almost equal motor? By choosing one with same motor constants kt- torque constant or kv-voltage constant. Note: these two constants are the same, just fliped arround, so if you identify one of them you are done.

kt - torque constant A/Nm. 

Mabuchi: 


stall current/stall torque = 85A/550mNm = 0.1545 A/mNm
max eff current/max eff torque = 10.1A/58.3mNm = 0.1732 A/mNm


At the stall point the kt tends to be lower due to core saturation, so compare whatever you have for equivalent.