The goal in rebuilding an engine is to return its performance and reliability to what it once had new. The goal in “building” an engine is to increase its power within the capabilities of that engine without unduly ruining its other performance factors — drivability, mileage, reliability, and perhaps smog-law compliance.
Building a performance engine is not just a matter of tossing “speed parts” like a big cam into it, nor is building high performance or racing engines anywhere near as simple as some people imagine. Change one thing and that’s a different engine. Change many things and you have entered another profession, as an engine developer.