You need a tensioning spring and adjuster, do you have that? Also need the bolt that attaches the drive to the engine block's mount bracket. There is a notched bracket on the engine that limits the forward-aft movement of the mule drive; the tension spring fits a notch on the drive arm and the adjuster fits the front "bumper" of the tractor frame. The drive bracket pivots on the bolt and there's a grease zerk to keep it lubed. Not sure about the size of the bolt, but I'm sure its not something you can't pick up at the box hardware store. It only has threads near the end and is a slide fit in the mule drive bracket. I've never had my drive off on either my 212 or 214, but it takes a 3/4" wrench. Note also in my picture that my tension adjuster ist kaput, I've got it held by a nut, which does work but requires a wrench. The proper adjuster has a small "T" handle so you can tension the spring by hand.
Hope these help.