This is actually a fairly difficult problem, it's something I have struggled with myself. Simply taking the old melodies and assigning new sounds to the tracks doesn't work very well, even if you have a vast library of software instruments (which I do). Often to get it to "sound the same" requires actually changing the melody or harmony lines.

Here's a link to one of the old tunes redone with native instruments synths: https://soundcloud.com/music-by-talin/faery-tale-adventure-nighttime