Not sure you have a valid option other than rejetting the carb. Your bigger exhaust is allowing more air/fuel out but you have not set up the carb to allow more air/fuel in. Exhaust changes and jetting go hand in hand and both are required to get optimal effects.
If you think of the engine as a pump you had a pump with a garden house for the intake and a garden hose for the output so you had great pressure and balanced in/out. Leaving that garden hose input and adding a 2” output means you will not be pumping enough to get the same output pressure because your input is now restricted compared to output.
Jetting is simple and not a permanent change.