Jack,
The motor has a shaft which is obviously a lot smaller than the Skywatcher motor,I am thinking the shaft’s length will be near on the width of the belt so I will use the cover as a stop to keep the belt in place or failing that I will make a sleeve to slide over the shaft so as to make it longer. The motor itself will be easier to attach to the custom cover due to the bolts being at the rear of the unit (aiding stability of motor)
Another alternative to the Skywatcher from JMI is just as bulky and same price as all the JMI motor focusers (£185) is pictured below, but not sure if the width between mounting bracket forks will clear the belt so will possibly need modification with a file. Just to note the Skywatcher focus motor is only £45 – £50 if the bulkiness of the mod isn’t a concern.
Best Regards
Alun