Scooby, here is what I did when I needed to adjust my slip clutch and couldn't get to it because of that stupid, ridiculous outer plastic shield.
I took a pair of tin snips and cut that outer shield lengthwise from back to front.
I removed the four bolts that hold that shield to the gearbox.
Then I pried the shield off (using the slit that I cut to slip it over the PTO shaft).
I adjusted the slip clutch. (In your case, replace the shear pin.)
You can the replace the shield by using the slit to slip the shield back on over the PTO shaft.
I haven't replaced my shield yet just because I'm the only one that uses the cutter and I NEVER get off the tractor with the PTO engaged, so I'm nowhere near the PTO shaft when it's turning. (I'm not advising you to leave yours off.)