What a great machine! The slow speed could be because of the foot control. To fix it, you'd probably have to hard-wire a new electronic foot control directly into the motor. Or, it could just be that the motor is tired. To tell which one it is, you'd by-pass the foot control. If the motor runs full-tilt with the foot control out of the way, your motor is good, and your foot control is bad. ...and vise versa. Brewer makes a small belt specifically for the older Berninas, part #305.163.03, but I don't know that it would fit that particular model. I like to have the machine in front of me and try a couple different belts to see what works best. It could also be that the belt is too tight -- that will also slow a machine down.
Good Luck!