very easy Brent,
roll on down to your Yamaha dealer and tell him you want a sprocket for the good old Ag 100.
available in either 47t or 51t in good old hardened steel.
only catch is, the MX/YZ style has a 6 bolt pattern while the Ag has a 4 bolt pattern.
bolt P.C.D. is the same and so is the centre hole and dish.
i just bolt the 2 sprockets together then drill through the 6 bolt one into the other. a 10minute job with the right drill press.
thats what i use on my little MX125 and it's the same rear hub as the YZ125c.