I love to work on cabinets!
Machines? Heh! Not so much, but I do it to the best of my abilities, and when I get stumped I apply my favorite repair technique! "Honnnnnnneyyyyyyyyyyy!!"
To answer on the slide plates. For the shuttle machines, most of those front plates that are missing are because they were removed and replaced so often that they wore down the metal and became loose. When the machine was put down into the cabinet, the plate fell off, no one noticed and it got lost. With the 66 and 99 machines, it's because of the same reason only it was because someone tried putting it on the machine from left to right instead of the reverse...it doesn't "catch" and so falls off, again, unnoticed.