OTOH, mounting a large armature from a 5 hp
motor in the lathe won't be a small job.

If there is a live center dimple then you can proceed as in that
article. However, suggest you match the diameter of the shaft to be
pressed with the correct size drill bit before you start the drilling.
Wire gauge sizes are useful for that purpose. A dry run in the lathe to
confirm correct size hole for pressing would also be prudent. I used a
precision shaft with matching drill bit and precision reamer since the
extension shaft also became the motor shaft.