I could trade for a Honda CR250 single if required. I’m in the UK.
I wouldn’t have thought a 258 V-twin would sit very well on a Superkart. Be interested to see some pictures.
If your regulations require that an engine is CIK homologated then you wouldn’t be able to race the 258. For instance in the UK it wouldn’t be allowed. You should confirm this with your karting association.
I’m guessing that your 2008 chassis will be an Anderson Mirage (2-bearing axle). You can use this with a single, but it’s a little soft. Also there may be an issue with the curved tube coming back from the side rail being too close to the clutch case (CR250), although a Rotax 257 single will fit no problem. The Anderson Maverick (3-bearing axle) is better suited to a single. If this leaves you with doubts about your chassis, I can give you more detail. There is nothing particular to look out for but you should check that the chassis is flat and square and check around all the tube joints for cracking.
Here’s for an image of a bare Mirage Chassis
Apart from the 3-bearing axle, the Maverick is easily identified by the chassis extensions that carry the bottom ball joint, on the Mirage they are tubular but on the Maverick they are a solid construction see image below of a Maverick.