RE: run-in engine on a pipe?
You can run-in the engine on the pipe, as others noted.
Key is getting the engine up to operating temp, up to desired rpm (pipe rpm), and allowing it to run just slightly rich/cooler at that desired rpm.
The difficulty with this.is sometimes ...
You would want to have the pipe set up and know approximately what RPM your target is.
Generally not a problem. Starting with the pipe set a bit long is best. Get some time on the engine, prop for your target rpm (will likely get there with a light prop even if the pipe setup is little long). This will give you a broader needle valve range and that helps out a bit.