I built my own timing pulleys using the Grizzly mini mill and a dividing head. To determine the OD of the new gears
to be cut is simple if you have a gear of the same pitch to go by.  The 9x's use a XL pulley system so that makes
it a lot easier.
To calculate the OD for a new pulley first measure the original XL pulley which is a 60 tooth pulley with an OD of
3.79".  Divide the OD of 3.79 by 60 ( tooth count ) and this equals  .06316   Take this number and multiply by
how many teeth the new gear will be . In my case I decided on an 82 tooth gear.
For the 82 tooth gear  I turned the pulley blank to an OD of   5.18"  ( 3.79 / 60 = .06316 x 82 = 5.179 )
This was the largest OD I could use without modifying the gear terrain cover.
For the smaller 18 tooth motor gear I just measured the OD and copied it directly to the new gear which is also 18 teeth.
May not be the right way to do this but it works well.

              Stock 9 x 20  60 tooth gear on left and blank on right             The 3/4" thick blank it turned to an OD of 5.18"
 
 

                         ID bored to 1.1800"                                                                Completed gear blank
 

           Grizzly mini mill and the Grizzly dividing head                         I used a 1/4" lathe bit and ground the tip to fit the
                                                                                                       pulley grooves.
 

                       Cutting the new gear                                                           Cutting the 18 tooth motor gear.
 
 
 

   I used two washers pressed onto a shoulder to make                               Washers pressed onto gear
   sides or guides for the smaller gear.