I do not know how much demand there would be for the inserts but a high quality red triangle insert would probably sell pretty well because not many high quality inserts are available.
Brevet crowns would probably sell pretty good too for older models that came with them originally because there are not many genuine examples left.
Dash crowns would be good because they were used for 25+ years and new genuine crowns are now going for $100+ USD and used examples are usually worn out. They would also be better on genuine watches than a worn out original if they were WR to 30 meters or more.
The crowns need to have good springs in them so they will retain enough tension to click the movement between setting and winding position. I had some new TC crowns a few years ago and the springs quickly became too weak to push the stem back into winding position and the hands would turn when screwing the crown down.
Iirc most of the older genuine crowns had a stainless steel cap crimped over a nickel silver base. The nickel silver base part might be expensive to make today but I have used quite a few replica crowns with stainless caps crimped over a brass base and they seem to hold up very well. A solid one piece steel crown is another option similar to the later rolex monobloc design but it would not be accurate to the original design.
Good luck!