The engines are the reason why
Fifi, the Commemorative Air Force's B-29 in the US, is currently grounded. They've been donated/bought modified twin row jobbies but these have to be built up. Will mean more reliable and less trouble in the future. All comes down to money though as always.
http://www.cafb29b24.org/index.html
Another B-29,
Doc, is slowly being restored to airwothiness in Kansas I think. Can you imagine seeing two of these together? Schawing, the ground would throb.
http://www.b-29doc.com/
She's not really the last B-29 that can be made airworthy as Kermit Weeks has yet to decide what to do with
Fertile Myrtle but he's got plenty of other toys to play with.