With the motor being nothing more than an airpump, the better the airflow the better the performance, to a point anyways. A header with a better flowing exhaust will help some.

If you are going to piece-meal the build, I would do a header, with a hi-flow cat, and a turbo style muffler(the Raptor is a good muffler at $24). Once the exhaust is done, I would then install the Weber. After that you could upgrade the ignition, Msd works well w/carb'd engines. These are the basic power upgrades for the 22r w/o rebuilding the engine, borinmg the cyls., new cam. and over sized valves.