Swytch eBike conversion wheel axle diameter

Presumably you have a bike designed for quick-release wheels that have a 9mm diameter axle.

The Swytch eBike conversion wheel seems to use 10mm solid threaded axles that are secured to the fork with nuts - which makes sense as this provides higher clamping force required to handle the motor torque. (FWIW, this fact seems to be absent from the Swytch website, and it means that their product is not compatible with any bike.)

If your fork dropouts have sufficient material, you may be able to enlarge the dropout width with a file, but beware that you will not be able to use the 9mm axle wheel. It would be dangerous to do so.

You could also find a cheap fork designed for threaded axles. You would need to match your current fork's geometry (headtube to axle distance, rake). As threaded axle hubs are used on cheap bikes you will not find a replacement part available for sale as a separate component. You'll want to get one from a old used bike. A good source for things like this are bike cooperatives or bike stores that specialize in refurbishing old bikes. They often have piles of bikes that are not worth refurbishing or frames that have had parts stripped from them.