A few years ago my Seiko SKX007 stopped working. I did the obvious checks (rotor movement for winding etc), then put it into a drawer meaning to get it fixed and just wore my non-divers Seiko 5.
I know there is an active Seiko modding community, and I just became aware that a common modification for these is to replace the 7s26 movement with an NH36/4S36 one, to give the option to hand wind in addition to automatic winding, and also it's a hacking movement (you pull the crown out to adjust the time and it stops the second hand to allow you to set it precisely, whereas the 7S26 has a bit of a knack to doing that as you have to stall the movement with a delicate crown operation so I never bothered).
Anyway, decided to give this a go, and the cost of tooling up (hand puller and setter, dial guard etc) was less than having somebody do it (about £80-£120 to have it done by someone, including parts). Remove the back, remove the crown, drop the mechanism, pull the hands, remove the dial, remove the day wheel (as the new mechanism was set for 3 o'clock crown rather than 4 o'clock so the day would be slightly misaligned in the window), swap the day wheel onto the new mechanism, then rebuild. Also needed to put a new crown in, as the stem is different on the NH36, which meant some careful cutting down of the new stem and cleaning the threads to go into the new crown.
About an hour of fettling, and the watch is running again. Now just need to see how it keeps time and regulate it if needed (I'd got the old mechanism to within a second or two a day). There is now an open source timegrapher application (Tg -
https://tg.ciovil.li/ ) to save some of the "tweak it and run it for a day" that I did with the old one.