RE: OS MAX .25 carb/throttle action
That is an RC carburetor. The "brass colored" object is the end of the throttle barrel, and it is supposed to be able to rotate. The black screw holds the throttle arm onto the carb. Here is the part number:
OSMG9000 22081408 THROTTLE LEVER #1A-3A US$2.29
These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.
If the throttle barrel is frozen in the carburetor, it can be freed by using a heat gun to warm the carburetor body. The carb body is aluminum, and the throttle barrel is steel. The aluminum will expand more than the steel, and so it should become easier to move and remove. You can also soak it in rubbing alcohol for several days to see if that will loosen the congealed fuel residue, which is the likely "glue" holding it all together.
If all else fails, a new carburetor would have to be purchased. Depending upon exactly which OS engine it goes to, one of the modern carbs may work, or you may be able to locate one through one of the internet hobby sellers or auction sites.