The amount of heat removed from the radiator is directly proportional to the amount of air passing through that radiator. Maximizing the air flow with the dirty aerodynamics and average speeds of shifter karts requires a single core radiator positioned straight up. That is because straight up builds the most air pressure in front of the radiator to push air through and a single core has less air flow resistance than multiple cores.
If your shifter kart radiator must be tilted back, install louvers on the front of the radiator, pointed forward and sealed against the sides. A small improvement in heat transfer can be made by adding a forward facing 1” lip sealed against the sides and top of the radiator. This last technique by itself is usually worth a 3-5 degree reduction in water temp.
This all begs the question what size radiator will be required to cool the engine to 120 degrees on 95-100 degree days. On days like that a high performance SRS stock CR125 will require a 10x17 inch or larger single core radiator.