The nice thing about Max Gray and USH steam engines is they were built like tanks.
The MG engines did not have the nice lost wax castings that later USH and Westside did, but you can add them. As far as the drive goes , the early MG axles were 1/4 or .250 in. The later USH were metric, so if you want to change the gearbox to a USH box you also need a metric bearings and axles. The hardest part is quartering. I hope this helps, I don't claim to be an expert , but I just wanted share this.
The early MG axles will take the KTM gear box, the bearings & gear can be bored to accept the 1/4” axle,….however, it was always hard to find separate sale KTM gearboxes except on the secondary market,….in the case of the K5, I do remember swapping out the gear box on the example I worked on, ….more than likely I used a NWSL, as at the time, they were plenty available….now not so much….
Pat