You can buy the armor only and wear a standard kart suit over it. Companies like forcefield and others make nice armor for motorcyclists that can be reused.
Motorcycle incidents at the track also happen at higher speed, so probably the extra abrasion resistance is needed. Motorcyclists also wear mesh jackets, but these dont last after 1 high speed slide. I think these mesh jackets are similar material to our karting suits but with armor.
You can also get custom leather suits fitted to you.