I want more specs for this motor:
131:1 Metal Gearmotor 37Dx57L mm with 64 CPR Encoder
Specifically, does anyone know the torque and voltage constants of this motor, as well as the viscous friction coefficient?
We do not have those gearmotors characterized in that way. However, you could estimate various motor parameters using the specifications on the page that you link to. For example, you can get a general idea of the torque at various speeds by approximating a linear relationship between zero torque at full speed and the torque at stall. The torque and speed will vary proportionally with the input voltage.
Would it be possible to order a custom version of this motor with a higher CPR encoder?
How many CPR are you looking for? Please note that the 64 CPR corresponds to 64 counts per revolutions of the motor shaft and that this will give you 64*131 (or 8400) counts per revolution of the gearbox output shaft.
We might be able to look into this if you are trying to get a lot of these. If this is the case, please email us directly with the motor specifications you want.