I saved weight by shortening the chain
Seriously. I looked at Moto3-bikes. They have a very short wheel base, the rider's butt is almost above the rear wheel center. Shorter wheelbase can't hurt on go-kart tracks where the bike does most of it's action. My son maybe better able to keep up with Supermotos and Mini-GP bikes.
Racing circuits in Finland are on average slow speed with very little fast turns.
Google says that a Moto3 bike has 17cm shorter wheelbase than a stock Ninja.
I grinded the swing axle holes a bit to move it forwards enough.There's now 9mm between tyre sides and swing arm.