The essential inspection, to the point that it would make all other assessments useless, is to make sure that the engine is assembled only with the manufacturer’s original parts. It is a recommendation which applies to all components, with extra attention to the connecting rod and the piston. This is not due to particular technical reasons, but to the simple fact the these are the two components with which self-declared engine builders or improvised mechanics “experiment” more, with the sole result of compromising the reliability and also the performance of the engine. If you have the opportunity to check the inside of the engine you must always look for the logo with which any manufacturer brands its spare parts. If you do not find it, be wary. Remember that the best and only guarantee when it comes to a kart engine, is the incessant development and quality control checklists undertaken by the manufacturers in producing every single component.