Convert a go kart into a race kart?

Most racing leagues require you to use specific parts on your cart depending on what cl it is, so building a custom kart is not allowed in most leagues (the official reason is safety). So first have a look online for your local karting clubs and find out what you can and can't use. Having said that I can recommend at least getting a proper chis as they can be surprisingly difficult to build and design well enough for what you'll want to upgrade on your kart.