Check pages 76 and 77 of the CVP-305/303/301 Owner's Manual, about "Changing Pitch," including scale tuning. The CVP-303 has nine preset scale types; there's a table on page 77 that lists them and shows how many cents sharp or flat each note would be if C were used as the base note for the tuning.
Some brands of keyboard (such as Roland) have an option for stretched tuning, but I don't see that mentioned in the CVP-303 manual.
I know that on the next model up (the 305) the stretch and flat tuning options are in the voice edit mode. That said I would say as the poster above to make sure to check the scale tuning in case something got messed up. The stretch tuning on my CVP-305 isn't too noticeable except in the last five notes at the top of the keyboard or so.