You are supposed to change the whole assembly, that is the drum/roller assembly. If you look at the MBK S 3800 parts catalogue, on the galet-embrayage page (roller-clutch), http://solexin.free.fr/medias/acrobat/sws_doc_solex_3800.pdf ,the individual pieces aren't even listed, just the whole assembly, 61287, rep. 1 in the diagram.
To change just the roller is not the easiest job, and you need an eight-point, thin-wall 37mm socket to loosen the drum nut. (see attached photo)
I don't know the projected life, but I've never had to replace one. If you use incorrect tires, you will wear it away. I've read about one that cracked into a few different pieces and was glued back together and had some carborundum grit glued to it.http://www.users.globalnet.co.uk/~pattle/nacc/arc0108.htm
So, I recommend swapping the whole drum/roller assembly. It will be very easy to do.