The idle mixture screw is retained by a spring that's on the nut:

OSMG9072 22481420 THRTL LVRNUT #2CAF60 US$3.99

That's the nut with the spring.

Here's the needle:

OSMG6424 22481628 IDLE SET SCR #2B2CBD US$4.49

These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.