I decided to change the motor to another one I had, however this one had a 5mm shaft. I had previously used a spur gear from the Jato on my Traxxas slash so thought I would see if I could do the same on the Pro-2. However, to do this, I need to use the out-drive shaft from the slash that the Spur gear mounts to as the spur gear is held on with a different method (small pin).
On the left is the Pro-2 gear box, on the right a traxxas slash gear box. The main diff gears are basically the same on both - same amount of teeth and same pitch, so I decided to leave it in there. I swapped over the two gears on th top left of the image below and it seems to work perfectly. Meshes well and is nice and smooth. I did have to add 3 small washers to the longer end of the shaft to get rid of a small amount of play.
And with it mounted
I've used a 56T spur as this is about as big as I can go before it'll start rubbing on the axle. Paired with a 16T pinion.