I didn't know the game Platypus before, but it looks like every one of its level themes is a very close remix of a famous C64 tune.
1) Comic Bakery, Martin Galway
2) Parallax, Galway
3) Sanxion, Rob Hubbard
4) Aztec Challenge, Paul Norman(?)
5) Wizball, Galway again
6) Driller, Matt Gray
The original C64 tunes are all awesome and really worth checking out.