That's a Magnum GP engine, which is the exact same engine as a Thunder Tiger. Global Hobby/Hobby People will definitely NOT be carrying the needle valve. Magnum and Thunder Tiger parted company several years ago. Tower doesn't stock parts for the Thunder Tigers either,
AFAIK.
You need to find a hobby shop that is an Ace Hobby dealer, one that sells Thunder Tiger products. The needle valve for a Thunder Tiger GP25 will be the exact right one for your Magnum.
If the needle valve fell out, then there's something more wrong with the engine than just the needle setting.