Comment by yobbo

> It loses about 5 minutes a day

This suggests it's completely out of spec, and maybe beyond saving. However, regulating functioning Seiko movements is certainly within reach for enthusiasts using a timegrapher device, or software with a microphone. Persistently adjusting over a few days, it should be able to get within -10,+10 seconds per day.

The timegrapher will also reveal the condition of the movement and whether further work is worthwhile. Servicing these movements is likely more expensive than replacing them.