If it means so much to you in terms of sentimental value. Me personally I would send it to Longines.
They actually are quite reasonable in their prices.
The only thing I would say is it would probably be a couple of months to do it as they are on a backlog. I know because they still have my Heritage Diver 1967 from the beginning of June lol.
It's worth the wait because they do a brilliant job.
Acorrding to the current price list it's £180.00 for a full service & £55.00 for a partial. Sent from my SM-N960F using TZ-UK mobile app