Brian hits it right in the nail, couldn’t said it better myself. You will get frustrated from wrenching from one set up to another ( especially at the track ). You don’t really wanna do this at all, I can understand when people blow up a motor during a race day, or get a top end stuck, it is very doable to replace engines at the track, or repair a top end, as long as you are replacing exactly the same engine package and new parts, but a whole complete and different engine type and drive asy. NO WAY!!!!!!! ( this is something you wanna do at home, before track day)
I am a certified technician (mechanic) by trade, and we have about 7 different karts we currently use, and I’m constantly working on them, and changing an axle to reposition bearing carriers to fit a 4 cycle engine, fitting the engine mount, and all components, etc is not something you wanna do in between practice sessions.
You really wanna spend more time driving, getting some seat time, and getting familiar with the way the chassis respond to any changes. ( no wrenching all day )
My best advise is: If you really have to have a new chassis with a Rotax ?, then just get it
But just know that old chassis win races too, You can show up at the track with an old chassis and kick everyone’s butt with the right driver
My other advise is: If you are like me ( always needing my fix NEED FOR SPEED ) You will always wanna switch from different types of power plats just for the thrill of it ( there is a reason why I have 7 karts, lol )
Then do your math correctly with your cash availability, and your budget, then you may just be able to buy 2 karts for a very good deal, use them for a season or so, get very familiar with the sport, and then graduate to a new chassis ( you really don’t wanna make costly mistakes with a brand new chassis, or go off road a lot with that beautiful set up )Not to mention you can total a brand new kart really quick by bending a spindle, or having contact. I doesn’t really take that much.
Most chassis out there are still pretty decent and also have the modern/new style body work, you can always install brand new body work or powder coat the chassis, and it will look brand new again.
My 2 cents
Hope it helps