SATA is SERIAL ATA and PATA is PARALLEL. Serial is the more modern one its probably going to be trickier to source something using Parallel.
Without seeing some images its not possible to tell. If someone is brave enough to take it apart or can find a good picture that shows the hard drives ports and serial numbers it should be easy enough to see if sourcing something with same dimensions and port is possible. I would imagine you should be able to upgrade it up to 500Gb which is as big as they make the really small hard drives as far as I know.
If you were really good it should even be possible to knock together an SSD with the same interface.