I added an SSD over half a decade ago, that's why I was absolutely fine with it's performance (at least after moving to linux and a minimal WM, OS X died for me long ago from bloat)... it's only the human interfaces that started to physically fail.

I know you can pick up spare parts for these but the problem is that they are all as old (they are commonly advertised as new on ebay, but they are not), they are used parts, and it is quite a lot of work to get all the guts out of the top case, only to replace it with a slightly less used components.

The best middle ground I could think of is to selectively replace rubber domes as they fail by cutting out the bits of silicone and gluing in new ones... only needs to pop of one key at a time.