I'm looking for a children's book I read in the late 80s, early 90s. All I can remember is a boy and the little alien find the metal umbrella in the gutter, which turns out to be his spaceship.
Well it was written in 1957, way before the time you read it, and details aren't quite what you said, but this might be "Rusty's Space Ship" by Evelyn Lampman. The boy is building a wooden space ship in his garage, when a girl who is a friend of his comes by.
He was nailing on a piece of metal, but she couldn't imagine where it had come from or what its former use could have been. It was perfectly round and about four feet across. ... it was flexible, for as she watched, Rusty bent it easily to reach down over the other side of the ship.
When she asked where he got it ...
"In the city dump. Pete and I rode out there on our bikes yesterday to see if we could find anything valuable, and I found this. Somebody's thrown it away, so I brought it home. ...
The alien, who is named Tiphia, shows up. It has forgotten where it is from, so it takes the children on a tour to visit the moon and several other planets. Oh, the boy's pet dog also goes. Finally it remembers it is from the Andromeda galaxy, and returns the children home.
If you remember the alien giving the children oxygen pills and temperature-controlling pills, this is the book you read.