. . . I'm also looking for an idea to replace the missing the plastic Asteroids, Missiles and Torpedoes with. . .
You could try Litko Aerosystems space gaming tokens. I personally ordered a custom set of the Space Torpedo Tokens with sequential numbering.
You could, as an alternative to tokens, make your own asteroids from expanded polystyrene balls (use your fingers to pull off bits to make them rough looking), paint and mount them on flight stands.