There are ICC’s (also referred to as KZ’s as I recall) which are purpose built kart engines from Italy, from a variety of manufacturers, which also run to controlled specs, on race gas. There are also modified moto’s like the Honda CR125. Modifications could include open porting, ignition, carb, pipe, etc, also on race gas. These two engine types typically run together as one class.
I’m not that familiar with SKUSA rules, but you could look at the WKA Tech Manual, in the Sprint Road Race section, under CIK or WKA 125 Shifter. That will give you an idea.
Generally open/KZ shifters are not that popular in the US. However the Northeast Shifter Kart Series gets a good number of them, but I believe they are mainly ICC’s.
Maybe one of the NESKS guys could chime in hear with more details.