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.

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!