How to fix the brake on Technic sp10 mkII?

If I'm thinking of the right fastener,it might be called an E-clip,or E-retaining ring Some Sears hardware only stores might have them,Lowe's etc.And best of all,a real private owned real hardware store.

The key is that you say you have the part. If it is undamaged (check out the felt liner), then it can be easily reinstalled provided that the two posts (one fixed and one variable for brake tension control) are still present in your unit. If those are missing, then you have a problem. If those are present, all you need is two small circlips which fix the brake liner in place on those two posts.

Take off your platter. Then remove the teardrop shaped metal piece that surrounds the revolving motor element, by removing the 5 or 6 small black Philips-head screws that hold it in place. Once you do this, you will be looking at the channel for the brake liner, and you should also see those two mounting posts that engage the metal ends of the brake liner. The felt pad goes against the revolving motor assembly. The rest is obvious. You can probably get small circlips at any good hardware store.) I just got thru reinstalling my brake liner today after cleaning the bearing well and replacing the oil.