One word, DREMEL, very handy tool, can port match for inlet mainfold , remove dags inside the case covers , use as polishing wheel... I got one for my CT70 im fixing up and damn its done alot of work , best $100 ive ever spent.
crazzy are you talking about the countersprocket when you refer to the cog? if so, I use a jig saw with a metal blade, just carefully mark the cover on the cut side and go at it, it works well, then you can grind it with the dremel to clean it up.
For my first one i used a grinder, delicate but did the job! Then for the second one i purchased a Dremel from bunnings for $50, did the job perfectly! Also used this for porting and polishing.