Welcome to the site !!
As a owner of a 2006 450X and a 2020 450R , I can say without a doubt ( note that I do not Supermoto , just talking about stability of the platform) , that if you were to go the route of a Honda for Supermoto , then the X would be the bike to do it on .... the bike is very stable and for transitioning from asphalt to dirt , it is the only choice of the Honda lineup aside form maybe the 450L .... the new Gen R or RX are twitchy , and a handful no matter what you do , I have already added a Flywheel Weight and changed the gearing on my 20 , just to make it easier to ride , I went from 13/49 to 14/52 gearing to tame the beast on dirt , along with a flywheel weight because it has so much inertia in transitioning from on the gas to off that it upsets the bike and makes you feel like you are going to go over the bars when slowing down for the entrance to a corner , especially if you drop to 2nd gear .... in my opinion ( and I am a Honda guy) the new Gen R/RX would be the worst possible choice for Supermoto ...... and as Neil mentioned , KTM does make a purpose made bike right out of the box
But if you were to go the route of the X , then what you would need , besides the wheel sets , is to lower and put limiters in the suspension , run a steering stabilizer ( unless it was one of the 08 on models) , and even with that I would stay away from the newest X , as it is basically a tuned down 450R now , and has some of the tendencies of the R and RX , so its going to be a bit twitchy , so the last gen would be the best choice ...... if you have not purchased a bike yet , you can find X's that have already had the conversion done , which will save you tons of cash ..... providing it was converted correctly as Neil eluded to ....
That said ..... if going the Honda route , I would choose a 05-17 X over the newest gen ....