So out shooting the other day the plunger assembly on the side decided to go MIA.  It's gone.  I see you can buy replacements online.  Are they all a standard size?  Are they a PITA to install?

They are pretty standard, assuming you buy a quality brand of replacment, and assuming that Remington builds their frame with standard hole size and spacing.

You will need a plunger staking pliers or a special staking punch and a piece of drill rod to fill the tube (to prevent crushing) when staking the legs of the tube.

I had one come out of a Colt "Enhanced Commander" so don't feel bad.  They are easy to replace.