Ditto mate , exactly the same happened on my son's RM125 ('03) yesterday. I have split the cases and was well glad I did. All the bits of plates were ground up in the tranny, good job they are aluminium and don't seem to have done too much damage.
I would advise doing the same though, all the bearings and oil ways were full of swarf, and whilst you could be lucky and get away with lots of oil changes, I would rather be sure and strip it right down and clean it out.
I will be drilling the inner clutch hub to improve oil flow and cooling to avoid the problem in future, search this site for 'drilling clutch hub' or similar for others experiences.