It's pretty unlikely that you would ever need an entire score reel assembly on a restored working game. The only part you're likely to ever need is a coil, maybe (rarely) a switch or coil stop. I don't think any of those items will ever be unattainable. If you can get them for about the price of a new coil, they would probably be worthwhile.

If you have an un-restored game with questionable score reels (or a possibly broken plastic reel) they might be more valuable to you.