Could just as easily be the PTO safety switch. You can replace it with a new one or, if you prefer, bypass the switch. My MTD tractor needed two of the three safety switches dealt with this way.
I recently acquired this 20 HP 46" Yard Machines tractor and am in need of a model # to make the right parts purchases down the road. In addition to the pictures below, The Briggs and Stratton engine numbers are as follows:
Model 461707
Type 0143 E1
Code 9912085A
An MTD agent told me those...