Is author Ken Kesey's ("One Flew Over the Cuckoo's Nest") bus Furthur still roadworthy?

No.

"The original bus's last journey was a trip to the Woodstock Festival in 1969. Once its historic trips had come to an end the bus was gutted and used around the Keseys' farm in Oregon until at least 1983, when it was mentioned and pictured in an article in the May-June Saturday Review. It was eventually parked in the swamp on Kesey's Farm, where it deteriorated over the years."

https://en.m.wikipedia.org/wiki/Furthur_(bus)

The bus was removed from the swamp in 2005 and is now in storage. A foundation has been established, furthurdowntheroad.org, to fully restore it (to commence once sufficient donations have been received).
http://furthurdowntheroad.org/furthur-the-bus-then-now/

Video of the bus being pulled from the swamp, in two parts:
https://www.youtube.com/watch?v=W4MxSv0oZH4
https://www.youtube.com/watch?v=NKg_XJHzKVQ

Picture of the bus in storage is on this webpage:
https://gorillagirladventures.wordpress.com/2017/05/27/on-the-bus-outriders-at-ken-keseys-farm-with-ken-babbs/