You need to decide if you are going to use the OEM replacement rivet that comes with the plastic sliding piece, or, whether you are going to obtain your own clevis pin and cotter pin.

The OEM replacement rivet needs to be peened over to secure it to the arm. But, if you peen it with the plastic piece beneath the arm, you'll destroy the brand new plastic piece.

You will have to take the plastic piece apart (it's a snap together affair) so that the underside of the rivet is exposed and can sit on the anvil. You'll need some washers under the rivet head for this purpose.

Then, you can peen the rivet to the arm.

I think the approach of a clevis pin with a cotter pin would be a perfectly acceptable alternative and would allow an easy replacement, in the future, if the plastic piece ever fails again. You probably would not need to remove the regulator on a future failure.

Be a bit careful of a standard nut and bolt. You can't tighten the nut too tightly because the plastic piece needs to rotate. A shoulder bolt would be ideal for this purpose, but, you'll probably struggle to find the proper length and diameter.