One of the most important signs your engine is working well is the Exhause gas temperature. This sensor will tell you if your engine is performing at maximum efficiency all the way through the race.  This is essential if you want to monitor your motor's performance
Buy an EGT Sensor here

Measures the speed of your kart to give a speed trace and also when compared to RPM will show you the amount of clutch slip you are getting.
Buy a rear wheel speed sensor here

Measures your air fuel ratio from exhaust gases called the Lambda ratio.  Use the information to study how your carb jetting is working well all the way round the lap.  This will show you where your engine runs lean and rich, and allows you to get your jetting perfect.
Buy the Lambda sensor here

This is the throttle sensor we use connected to an E-Box. We prefer this sensor to the usual linear sensors because its really simple and fast to fit to any kart, and takes it readings straight from the throttle cable motion. See an article on how we use this sensor here