Noob question here. Can a header of any size connect to an engine? I have purchased a 1915cc engine and recommended exhaust system is 1 1/2". If I purchase a 1 5/8" does it bolt on exactly the same? Any adaptors required?

The sizes your referring to is regarding the pipe diameter. The different sizes will fit but will change the characteristics of the engine. For best results, the diameter of the exhaust needs to match the rest of the components, ie. the engine displacement, heads, cam and carb size all vary what exhaust is best. Put on a too big an exhaust, and you'll take away bottom end performance, where you mainly drive.......

What Matt said is very true, tell us more about your engine.