What 60's pop song was based on a Mozart piece?

You may be refering to the "Lover's Concerto" sung by the Toys (1965).

No, that is based on a famous melody by J.S. Bach. There was a very minor hit in the early sixties based on the opening of the first movement of Mozart's Piano Sonata in C ("the simple sonata"). It even kept the trill in the melody line (4th or 5th bar?).

Unfortunately I can't think of the song's name. I think it made top 40, but probably not top 30. :) I only remember it (minus the title) because I was taking piano lessons back then.

Wish someone could remember the name though.