Is there a reason to remotor? What problem are you trying to solve with a remotor? Is the problem caused by the motor, the gearbox, or the mechanism (or some combination)? Replacing the motor where the gearbox has a lot of friction and/or the mechanism has binds doesn't improve performance.
Assuming the gearbox and mechanism are not issues, an old motor's performance (noise, heat) can often be significantly improved by cleaning the commutator, making sure the brushes make good contact, a small drop of oil on the bearings, and/or replacing the magnets with a rare earth magnet stack.
The closest replacement to what you have would be a modified Pittman motor design from Bowser. Shaft sizes would probably match up well.
Newer motors generally have smaller shafts. And their bearings are not as strong, so direct connection to the worm is rarely the best installation for a new can or coreless motor. A universal between the 2 is generally recommended.
The gear ratio may have to be altered to get desired speeds with the new motor.
Almost always, a new motor mount will have to be devised. Isolating the motor electrically from the frame will allow a future DCC install. BTW, brass locomotives generally benefit significantly from installing additional electrical pickups.
On the plus side, the relatively large size of the locomotive and the brass construction mean that it can be a real puller as well as a beauty with the right motor, weighting, and gearing. It should also accommodate a decent sized flywheel and DCC sound, should you so choose.
my thoughts, your choices