I've never owned a mechanical watch(did just recently buy an Orient solar watch after I lost my fitbit while traveling and after a couple weeks, realised the only feature I actually missed was having the time on my wrist), but I like watching the channel Wristwatch Revival on youtube. That guy will often repair old watches that most watchmakers won't touch, sent in by viewers, and make content out of it. And he especially loves taking on watches thathave sentimental value for the owner.
He also recently started a watch repair shop specifically catering to cases most watchmakers won't accept, might be worth checking out.
Tempting, but "Services start at $1,750" --- I could buy multiple replacements for that, or make a downpayment on the Seiko Grandmaster or GPS which I want...
That said, I'll keep it in mind and maybe if I come across a windfall will apply thusly....
For a Seiko, I'll also recommend https://www.theseikologist.com/. As his name suggests, he specializes in Seikos and has a large stock of vintage Seiko parts which are hard to source. Though, it's still not cheap. I have a broken Seiko chronograph from the 70s and he quoted $500 to just do the watch equivalent of a tune up. He can't give a better estimate without opening it up, which is understandable.
If it isn't a hardship, I do recommend fixing your dad's watch. Sentimental pieces are always special.