Karts have engine break in just like everything else, but because of how little power they have (and how few races you can use them in) don't expect huge gains.
The 125 SPLs can get 31hp after break in and the 100s can get 24-28ish....cant remember the exact number but its in that area. If you join a kart room and ask, i guarantee someone there will know, theres some big gt5 kart nerds out there lol.
Maybe you can making allowances for the theory of the structure of a kart, but you might acquire the minimum gain on the engine power through the procedure since they have very little horsepowers.