There should be a lever on the motor head
that can be engaged and disengaged to tighten the belt when using the autopilot and loosen it when you're not using it.
If you're engaging that lever and the belt is still slipping, then one of two things is the problem:
1. Incorrect installation
, the motor head
needs to be moved further from the wheel (note: if this installation
has worked in the past, this probably isn't the problem).
2. The belt has stretched and you need a new one. The belts can still be found online, but it can sometimes be a challenge. If I remember right, they made three different sizes, measure yours and start looking.