I agree with Denny, you need a seat cushion for the seat back for ultimate comfort. You can make one or buy one.
As for wanting a rack and pinion steering setup, I didn't read you describe a problem with it but is it because you don't like the effort it takes to turn the steering wheel? I told another member with a similar kart to make sure the steering system (all the linkages & component parts) are lubricated regularly and if there is any binding in the steering system, find the problem and address it. A lot of used karts spend their lives outdoors so anything that can rust usually does, and that means the steering can get stiff from the rust in the joints, kingpins and spindles.
If it was me, I'd take off those
ugly neon green 'pool noodles' and add black padding meant for UTVs/go karts....but that's just my personal taste...
P.S. I am not fond of the current tie rod ends on the kart, the ones connected to the wheel spindles. Prior experience with that type of ball and socket joint means that when it wears out, the socket pops off the ball leaving you with no steering. That could be a major problem if you were far from home on a remote trail. I prefer heim joints that have a "captive ball" type of swivel. Something like this:
https://www.bmikarts.com/516-24-Female-Tie-Rod-End_p_1475.html
Just something to think about upgrading.
