1680 red sub MK IV

I guess you are talking about a bezel insert, since you already have the bezel. They are readily available on the secondary market, you can search watchrecon to see prices. As you can guess, there will be a range depending on condition of the insert, condition of the pearl, fading, etc. But you should budget somewhere in the range of $2k USD to get something acceptable, give or take.

In addition to a folded 9315, you can use the updated 93150, which is probably a bit easier to find in good condition. You didn't say the year of your 1680, so we can't tell you which one would be period-correct. It's really a matter of taste, but bracelets are often changed out, and I personally don't think it's such a big deal. In fact, I have a relatively small wrist and don't really like the flip-lock so much, so I have sometimes worn vintage Subs on a 78360 bracelet. The important thing is to make sure you get the right end-links.

However, most importantly, as often emphasized on OF in the past ... don't source these parts, that's too fancy and expensive. Just buy them on eBay or in the classified section of a collector's forum. 😉