The typical design for the type of bearing on the motor you are working on is to use grease lubrication. The idea of the plug at the bottom of the bearing housing is twofold: to allow the old grease to be purged out when new grease is pumped into the bearing housing; and, to allow air to be forced out and a more complete or positive forcing of grease into the bearing.
We had this design of grease lubricated bearing on a number of motors and some generators at the powerplants I retired from. About the only cautionary note on this is:
Avoid over-greasing the motor. This typically occurs when an uninformed person sees the grease fitting on a bearing housing, puts the grease gun to it, and pumps in plenty more grease without first removing the bottom plug. The result is grease gets forced out of the bearing housing, around the shaft, and into the windings.
Typically, this type of bearing assembly is greased after assembly (or cleaning/flushing) by pumping in the grease with the plug removed. The critical thing here is a fine balance between pumping until a solid flow of grease comes out the bottom plug tapping (like toothpaste from a tube) or just seeing some grease starting to be forced out that tapping. We always went with the beginnings of a grease flow out the plug tapping rather than a solid flow. Our reasons for this were to allow for "working" and expansion of the grease when things warmed up in the bearing.
An important thing to do, now that you have the motor apart, is to wash the bearings, housings and all other related parts with solvent and remove all the old grease. The old grease may be caked, and you do not know the composition of it (Lithium based, or old soap-based grease). Start with 100% fresh grease. Without a hard specification, I'd use an automotive wheel bearing grease. For the loadings and temperature, automotive wheel bearing grease (also used on universal joints) should be fine.
