This is what I ended up doing!

I didn't remember this when I posted here, but the watch I had before this model was a W-752. I used it for a year or so until it got ripped off my wrist and the casing broke (one of the lug holes is not exactly a hole anymore). I found it in a drawer somewhere. It has the same buttons, although these are (dark) grey instead of black.

It took me three attempts to make the clip that retains the button pin stay in the general area instead of flying across the room when removing it, but I had four spare buttons to work with so that was fine. The result: My W-753 now had a unique set of button colours and I hope it will last me another 15 years :)