All this talk of WR450F or WRF engine doesn't make sense for the OP. I'm sure he meant a big bore version of the low maintenance WR250R, not the higher maintenance WR450F.
Sounds to me like the WR250R is pretty close, but if you want more grunt for highway speed, 690 Enduro or TE630 for modern, or DR650 for old-school. No kick starters on 'em though.
If you're willing to do a bit more the maintenance, the 500 EXC looks like a peach of a machine.
What it really sounds like you need is more time to ride!