I’m trying to find a book that I read in grade school, back in the late 1980s.  (Probably between 1985 and 1987, but it might have been as late as 1989.)

I believe it was a chapter book, not illustrated or with minimal illustrations.  I checked it out from my elementary school library, which had a tendency to rebind paperbacks, so I don’t know whether it was paperback or hardcover, or other details about the format.

The main character was a grown man who was turned into a child by magic, and had adventures on the back of an invisible brontosaurus.  At the end of the book, he returns to his normal age.

I feel like part of the story also took place in a museum, but I can’t be certain.  I also have an impression of a suburban street at night — also not something I’m certain about.

Thanks!  Hoping you can find this one, because I’d love to read it to my daughter.  (Assuming it’s as good as I remember!)