SciFi Alien rescue and adventure. Read 1970.

[Aleah]

a science fiction book read in high school in 1970

A man is walking in the woods and finds a tool that seems to be a flashlight. When he turns it on a laser beam appears from it. He walks further and finds a wounded man who he rescues and helps heal up at the man's cabin. The rescued man is really an alien that crash landed on Earth.
Using the laser device the alien builds a space ship to return to his home planet and invites the man to join him. The man does go with him and has many adventures. The successfully return to the alien's planet.

The spaceship is built like an onion. The fuel source is wrapped into the layers. They travel to Ganymede.