Most simply use clear silcon caulk, lay down 2 light bead's and set the motor in being sure to line up the motor shaft with the gearbox shaft, When your satisfided with the alighnment set it aside to cure for 24hrs
Its what i'm getting ready to do with a Tenshodo GN. 2-10-2 I'm remotoring in order to install a DCC decoder.
Micro-mark does make a universal motor bracket for locomotives you might be able to use,you can check their website under model railroad items just type in can motor and a motor they sell along with the bracket will come up.
One thing you what to remember brass steamers are lubrication intensive.
gearbox use labelles 106 plastic compatible grease,likewise the siderods need labelles 108 light oil.