The only reason to replace the poppet is if you break the top section that captures the little O-ring. If you are careful, you should not need to replace that part.
That O-ring with the poppet works as a small piston to increase the reserve pressure as a function of depth. There is a small hole at the top of the valve where water enter and pushes on the top of the poppet.
When you unscrew the top cap on the valve you just need to be careful not to put any bending load on that part. Soak any parts in vinegar if they are hard to take apart.
I have one of those poppets in my parts kits from the 1970's, but that is the only one I have. I still have two of those valves that occasionally I think about putting back on a tank. I used to really like those valves.
Note: the reserve section of the valve is not pressurized when the valve is shot off. Therefore, you can work on the reserve even if the valve is mounted on a full tank.
If you do break that little stem, there are a couple of work around that can be used to be able to use the valve. The depth compensated function may not perform as well, but the valve and reserve should work fine.
Good luck
---------- Post added June 2nd, 2013 at 11:36 AM ----------
I just thought of another easy work around if you break that stem, but hopefully you will not needed.
Here is a copy of the 1981 diagram.