I just "googled" motorcycle themed fabric" and found lots of choices.
You probably would feel better about making the quilt if you knew the brand of motorcycle and/or colors that interest them. Depending on the fabric you choose, it could be used more as an accent than a major focus. Something subtle.
I found some Route 66 fabric on fabric.com that I'm using to make my grandson a quilt for Christmas. It's the attic window pattern with the print in the window and three shades of blue for the windowpanes.
Let us know what you end up doing.