Found shop that sells rear dirt cover for medusas, wish i have found this before i sold my motor...
Anyway here's link.
They also have spare rotors 16,8mm and 17,3mm versions.
Drit cover? Looks like 5 cents worth piece of foam that you would find in a PC as a dust filter for the fans. Wonder if it'll melt if you got the motor hot enough, seeing as its in almost direct contact with the windings...
Looks like a scotchbrite pad to me. I use them at work to prep paint on cars. Easy to cut, they are breathable, and they should trap some dust. A box of 12 that are 4"x10" is 20 bucks. So you could make almost enough to cover medusa's complete motor production out of 1 box.