On the topic of what to replace and not, the dial, bezel and hands are a no-go. Be very, very clear they shouldn’t be replaced. And don’t get the case polished either. In general keep it as original and close to your grandfather’s watch as you can. Maybe polish the crystal, that’s up to you and the watchmaker. As for the movement, don’t expect parts to be easy to find or cheap. You gotta be patient and willing to pay to get this watch going again, but it mightn’t even need any parts if it was looked after by your grandfather. But seriously, what a watch!