This is a tricky one. First the easy bit. 'Clavier' is 'keyboard', and it includes harpsichord, virginal, spinet, clavichord and organ as well as piano.
Musical purists would regard 'classical' as being restricted to composers such as Mozart and Haydn, but not Bach or Rameau (who were baroque) or Debussy or Elgar (who were romantic). But most people nowadays would use the term 'classical' to include all the above composers as well as others ranging from Lassus and Victoria to Poulenc, Messiaen, John Cage and George Benjamin. They would not, however, normally include jazz or rock under that heading.
'Classic' does not, however, necessarily imply 'classical'. It tends to be used to mean something like 'renowned' or 'paradigmatic'. So, in BE at least, I think the best translation for what is required here could be 'classic keyboard works' - it would encompass all periods and genres of keyboard music. (In BE the adjective 'standard' is more often used to refer to jazz than to classical music).
