We raced the AMA modified production streamliner class with production engines when they were legal, in both 175cc and 1,000cc classes. The carb body needs to be the same as OEM on the outside. The trick to building a production engine is to realize this is the limiting factor - the amount of air that can be sucked through the carb. This is the first thing to figure out. Second is to calculate the power you can get with this air. Third is to figure out is if this power will give you the speeds you need.
Its basic engine math and a good idea to do it before the build. We did this the other way, the expensive method. We built up to the motors to the limits of what their carbs would flow and found out how fast they would go.