If you’re buying new, the dealer will help you buy the right kart. If you’re on a 206, you’ll want a 4-cycle specific chassis. The 4-stroke engines have the clutch on the seat-side of the engine. Chassis for those engines have the seat stay and crossmember in a place that won’t interfere with 4-stroke engine mounting. The X3-4S is the current Arrow 4-stroke model. I don’t see them on Kartsport or Comet’s websites so they may have to order one special for you or they may just not have them on the website.
I’ve raced an AX9-4S for years now and they’re great. Very easy to work on and very competitive in 4-cycle classes..