I will be purchasing a ROK 125 engine and I am wondering what is the best way to select the proper radiator for this engine?
I would give thought to where you will be racing/driving, and what the typical range is for ambient temps. Restricting air flow through the radiator is much much easier than adding airflow, i.e. plan for the hottest temps you’re likely to experience.
Of course, there could be other factors to balance such as weight (if you’re trying to reduce weight), and aerodynamics (more of a concern in road racing).
Brand wise, there are plenty of great options out there: New Line, KE, EM, KG, etc.
If you’re looking to maximize engine performance, and anticipate ambient temps of 90*F or higher, then I’d just go straight for a “big” radiator like the New Line Big (290x430) or equivalent from another brand.