If you consider all the components that make up a racing kart individually, the engine is the heaviest. Around 17 - 18 kg on a KZ, a little less on an OK. In percentage terms, an engine accounts for more than 20% of the total weight of a kart, certainly more than the bare frame, which is around 18% of the total.
However, the highest percentage of total weight belongs to metal components and nuts and bolts: added, together, they account for 30% of the total mass. Clearly, only if we consider them as a whole: taken individually, no component has the same weight as the engine.
If you wanted to create a "ranking" of how much the various components of a kart affect its weight, the ranking could be summarised roughly as:
metal components and nuts and bolts – 30%
engine – 21%
chassis – 18%
fairings - 14%
wheels – 11%
plastic components – 6%