Why not just put the spring and thrust button back in? Any small compression spring that will fit the hole will do, and round plastic or Teflon material is available on Ebay dirt cheap. It's not always necessary to throw a wheelbarrow full of money at these old rigs! The original owners certainly didn't.
I think that's what I'm going to do...put those parts back in. Is it also possible to get rid of the fitting, and return to the grease cup and its cap?
Thanks.
-Kevin