From what one can see of Google Maps satellite view, the old road appears to be paved for at least much of the way.

E.g.:

https://www.google.com/maps/place/Th...pn7p?entry=ttu

However, it is narrow and looks banged up in places. But the images are old, so it might be in batter condition these days. Or it might be in worse condition.

Bring plenty of water for the long climb up to Thompson Pass. The first time I came down it it was quite warm on the east side. Then check your brakes before the descent. The first 4 miles averages something like 7.2%. The restaurant in Murray has a nice well head to refill bottles. I have gotten good sandwiches to go there.

BTW...When I stayed at Thompson Falls State Park in 2017, it had unadvertised biker spots for less money. They put you in the day use area overlooking the river if it's not being used. There is a nearby "ramp" down to the river. Nicest place to camp in the entire park.