William,
I have some K&B black carbs and some pipes if you still need them? For headers, I find the best ones are the car 180* headers.
They have a little less volume I believe, and that helps the return wave of the smaller volume pipes move more velocity back into the engine. Pipe Volume is critical on our little engines.
Just a FYI, the stock Novarossi rotary .250” bore carb is still one of the best carbs on a 12-motor. I have over eight different carbs on my collection of 12-motors, and and I always go back to the stock .250” carb.
Just Let Me Know If You Need Anything,
Mark