A five pole armature and single start worm was the way to make this old open frame design deliver a better performance....I am sure that somewhere I have seen reference to a 5 pole armature for the X03 / X04 : quite an improvement in performance but perhaps I'm just dreaming...
In addition to retrofit armatures, both Romford and MRRC produced five pole clones of the XO4, and Airfix GMR had a Chinese manufactured version of the MRRC design (thus a 'clone of a clone') in their N2. I still have a couple of the Airfix GMR motors, and a Romford 'Bulldog' running well on the layout.
But modern can motors on multistage gear trains are better by a country mile: quieter, smaller current draw, more torque. Your choice, but I wouldn't fiddle around upgrading old motors, instead spend the cash on moving to the new and much better.