Hi - this is not a "generic" bid like others!
First, I'd like to know what kind of "counter" do you need: a counter that is on a car, or one in a central point, that can track ALL karts?
An in-car lap counter can be quite complex depending on what you need - this can be very complex like a rallye car telemetry system, which can even track and transmit ECU specific data like engine load. Or a very simple one that just counts laps.
Depending on exact specifications, project can be either very simple (an in-cart counter that only counts laps) or a central point one that can track all cars, their lap times, etc. Each prokect and approach requires the use of different type of sensors, from a simple hall effect sensor to a lidar.
This is a very exciting one for me, as I used to program ECU's decades ago. I'm also a mechanic, worked on marine engines for a few years. So let's discuss and build a great device!
I have a small home lab for such projects in my home, not only I have various microcontrollers like Arduino, but have very special sensors and equipment.
Best regards and looking forward to hear from you,
Baris