What I did was use light aluminum tin foil (kitchen stuff) as a build up layer. Only because there is very little pressure here involved to holding the brushes in.

One layer is usually enough to tighten up the worn threads.

Do not overtighten, which may have been your cause for stripping the threads.

Teflon tape will work for you as well, and or light copper foil.

Even the tinfoil from  a cigarette pack will work, but it is awful light and would be my last choice.

Do not do this if you have any tool with metal housing. You may have electrical shorting problems.

If it is an all plastic housing and plactic screws you should be safe to go.