Just west of Prineville OR, SR126 merges with US26: continue east on US26.
At Austin Junction OR (just east of Prairie City OR), leave US26 and continue on SR7.
At Baker City OR, SR7 ends: continue east on SR86, which becomes SR71 after crossing the ID state line.
At Cambridge ID, SR71 ends: continue east (and later north) on US95.
At Grangeville ID, leave US95 and continue east (and later north) on SR13.
At Kooskia ID, SR13 ends: continue east (and later north) on US12 all the way into Missoula MT.
Whether you complete the above by bicycle, motorcycle, or automobile, it's all seriously beautiful.