I think that DR650 looks like a good start.
Another bike you might consider is the DR350SE. About the same power and weight as the WR250R, but the simple air cooled, low rpm engine design of the 650s. Also steel subframe etc. Biggest aftermarket tank is 4.25 gal, though I've read on here that the 6.6 gal XR600R tank fits without much work.