I see... Yes, if it's cramped, that is a pretty basic problem. To stretch, looks like you'd have to cut and extend 4 bars - right in front of the seat. The cables may be too short then, but it looks like you have a little slack - especially if you'd reroute them down the steering support between your legs. Speaking of which - there are 3 cables. 1 brake, 1 throttle, and it looks like the other was for a shifter. The happyscooter site said it had some kind of 4 speed sequential gearbox on it. Not familiar with that.
What are you going to want to use this kart(s) for? That may help decide which to fix first and what parts to find...