I also had my '55 Ford dash clock repaired by Williamsons along with some other dash instruments. My clock was an electric type, not the manually wound type, but I am sure they can do both.
They do good work if you are willing to spend the cash.
In the past I have also used Instrument Services Inc. for a '55 Ford clock quartz conversion. It didn't last long, but from what I have heard, the voltage reducer/polarity inverter probably failed (junk made in China). So I cannot necessarily fault ISA's workmanship, but I don't like their return policy. They wanted to charge another $110.00 to repair it.
Williamsons has lifetime guarantee.