I feel your pain.
I have a 648 whose top gear just disintegrated while my daughter was sewing one day. I have good intentions of fixing it some day but haven't made it a priority yet. In spite of that I still like them.
Nylon parts failing isn't uncommon for any brand of machine that age. One of the reasons old all metal machines are so loved by many. 600 and 603 Touch & Sews are all metal inside.
Rodney