Re: Remington 51 .380
I just got finished responding to a guy who wants to take his Model 51 down to replace the trigger spring. My first advice was to remove the grips before working on the gun and how to do that without damaging them. DO NOT PRY THEM - that is the way yours got broken in the first place!
So, OK, you have cracked grips and can only get the plastic part as a repro. So here is what you do. First, drive the grip safety pin (at the bottom rear of the grip) flush on one side. Slide the grip down and pull it off. Then do the same on the other side.
You now have the grips and the backing plates off the gun. What you will have to do is drill holes in the back (inner side) of the rivets so they can be compressed and removed. Don't just grind them off as you will need to replace them.
Remove the rivets and the grips from the backing plates and replace the grips, riveting the new ones back on. You might (and I say might) be able to find rivets the right size, but I doubt it. If it is not possible to rivet them, you can stick the old rivets in for show and glue them in, then glue the grips to the backing plate. Replace the grips the same way you took them off.
The reason for all that elaborate grip system is that there was a patent on the idea of holding grips on an auto pistol with screws, and one John Browning had it. John Pedersen, the Remington designer (yep, he of the Pedersen device) had to figure out how to work around that.