Lots of variety in Warner Valley -- dirt roads, open washes, single track, sand. KTM 990 will be fine for most. Might be a handful for some areas. Benchmark Maps are the best. Buy the Utah book, covers the whole state. Smaller detail not shown, for that you can try some Google Maps, or just go check it out in person. Nothing beats a first hand look.
Not sure of my schedule that far out, might be Spring skiing, might be working. Generally, there are at least a few people around in Warner Valley.