Hey Dude :o),
I got my self a 04 yz 125 as well, your the first person ive come across thats got one like mine :o).
Ive done a few mods to my bike all ready , i put a FMF Gold series chamber on it and a FMF shorty power core2 Silencer , which makes it go so much better.
In regards to the sprocket, it is a 48 tooth, I was told if i changed the sprocket to a 50 or 52 that ide loose abit of top end, but i guess it wouldnt be noticable :)
Twaaaaaant Twaaaaaaang