Thanks, judaslefourbe. I assume a KTM front hub would work...from which model if you know? This will get me a set of street rims for the EXR to swap out.
I have found the easiest way to use a KTM front hub or wheel on the EXR is to use the ALTA spacers from my EXR wheel and make a 7.0 mm spacer for the brake caliper. There have been several threads discussing options and I think the consensus is the caliper spacer is easier and stronger than the disc spacer(s). This is due primarily to the small bolts holding the disc to the hub. Obviously with a caliper spacer it means you need to unbolt the caliper each time you swap the wheel instead of just swapping the wheel.
Definitely do not just stick 7 mm of washers on each disc bolt for anything other than a test fit.
KTM changed front axle and bearing sizes a couple of times in the past 15 years so if you are using a KTM front hub or wheel you will want to make sure the axle is the correct size for the EXR; e.g. the 2014 that Judaslefourbe mentioned. KTM RFS wheels are also the correct axle size. Otherwise you'll need to swap out the bearings, spacer, and seals. Also, the rear wheels from most recent year KTM dirt bikes are a direct fit on the EXR, including the KTM spacers.
If you are looking for a 21" wheel to mount street oriented tires there have been a number of new ALTA EXR front wheels on eBay at good prices. If you're shooting for 17" wheels and a full supermoto setup then you need new wheels and other parts.