In the mid-80's I forfeited my fee for providing talent in exchange for 20% of future profits. Can I collect?

I have a written agreement and the writer/producer of this children's album had claimed that it wasn't selling well and that he hadn't made back his production costs. I have very good reason to believe this isn't true. His latest claim is that he doesn't owe me anything because he's now selling this music through on-line digital sales, as if our agreement only covered cassettes and CDs. He's constantly bragging on-line about the checks he receives for digital sales through CDBABY. Is there a way to make him give me sales reports so that I can decide whether or not to legally persue this?