The motors need to be disassembled. Matchmark each end with some masking tape. Be careful, there are shims at the end at the bearing where the encoder shaft goes. Keep the parts for each motor seperate.
Protect the rotor ith bubble wrap and tape. Have a qualified machine shop center bore on a lathe and ream for a press fit. You can use .250" smooth dowel pins x 1.25".You want the runout as small as possible.
You want them to stick out about .750-.800". Reassemble the motor with the proper shims. Watch those magnets, they'll take the skin off your finger if you're careless putting the rotor back in.