Here's som detail pics of my conversion...
What you need to do this is an extra gear-case half to cut out the bearing holder closest to the spur-gear, a 5x11 bearing, and some spacers (~8mm)
The last pic showing a prototype garcase that I made before.
On the left half i cut down to the same level as the rest of outside of the gearcase. On the right i drilled up to 11mm (used a reamer from the outside) The original inside diameter in the gearcase is 11mm in the location that the 5x11 bearing will sit. The bearing holder from the extra gearcase half is cut flush to the same level from the other side. The screws and motor-plate will center everything up nicely.
/Mr. Pink