I have zero knowledge about the Dunlop K-180, but may have a somewhat similar looking alternative for you, as long as you can live with narrower tires (4.00 is the largest, both in 18 and 19") In my opinion, a "period looking" XS650 street tracker actually should have narrower tires than modern flat trackers. Just view "On Any Sunday" once more
https://en.reifenwerk-heidenau.com/modules/news/96_K34-New-Dimension.htm
On stock 18/19 XS spoked wheels, I'd be inclined to run 4.00x18 and 3.25x19.