The reason I mentioned to remove the front brake system is because Yamaha KT100 or clutch karts do not run front brakes, at least most clubs don’t run them, and having no front brakes will really teach you how to drive karts, as they are not as forgiving as the shifter kart brakes. ( I understand you drive track cars) And you may be experience, it’s just that the reaction time is a lot faster on karts, and things happen a lot quicker.
You don’t have to sell the front brake system, just save them for the day you’re ready for shifter karts, then you can put them back on.
You can probably find a good used KT100, KPV,HPV, or even an older 125cc Rotax, or Leopard engine for about $700 to $900 complete.
I would highly recommend to start out this way, still get the thrill of going fast, being able to craft your driving style, etc. Put a season or 2 and then jump into shifter karts, by then you will have all the info you need about shifter karts, what to buy and what to stay away from.
Where are you located at? And What kart tracks do you have around you?