As long as it isn't leaking don't worry about it, they do get a little oil and then collect belt dust. Pull the clutch from time to time and clean it off. If you blow a belt make sure there is no chord wrapped up in the seal behind the clutch. There is no way to change it out without spliting the case halves. The "ring" around the outside sits into a groove inside the case, that is what holds it in place. If you cut it down you will be asking for more serious problems for sure.