rebuild kit # K7505 $21.32. You can get replacement floats and venturi's also. The base jet in those carbs is problematic. We remove everything from the carb, soak it, bead blast it and rebuild it. So far we have been able to guarantee our rebuilds with no problems.
Since you have the R57 on your machine the new Zenith replacement carb is # 13720, AEI here in Oklahoma has them in stock for $190.00. They also carry all the carb kits and magneto parts. 405-236-3551 talk to Mike.
On the engine drive page of our website, down the right hand side, you will find all the details for the Marvel and Zenith carb tear down, etc.
I have rebuilt carbs with metal filters installed in stock for $55.00 + shipping. As long as you have replaced your 5/16 fuel line with new and cleaned your sediment bowl, I guarantee it.
Good Luck