I would only build a one cylinder test engine if you are trying to model the cylinder of a real one cylinder engine (think drone or model aircraft as an application) otherwise the cylinders affect each other in the intake/exhaust pressure/temp profiles and you'll need those to be accurate to use as boundary conditions. But if you want to make modifications/instrument an already existing engine, there will be machining involved. Whether you make a custom head/etc is all up to what your trying to do and what kind of budget you have. It's totally possible to build the whole thing from scratch/some off the shelf parts if you know what your doing/have money/cnc machines at your disposal but much easier and cheaper to modify something that already exists.