The KTM or Husky 500s are getting down close to what a YZ250 from 20 years ago weighed. Would be an awesome sumo. Not 200 lb less than 600, but 350 lb less than 600. Would be a different world.
On the other hand, if you're going to do a lot of miles on pavement, you need a cush drive rear wheel or your bike will eat transmission parts. I don't care what they say, they're wrong if they say you don't. The KTM 690 and variants have it. Can't think of a non-KTM-family high performance single that does.
Unfortunately, the company that was building cush-drive rear hubs for any dirt bike you wanted seems to be out of business.