There are quite a few synth tracks in both those pieces. But I really think you need either a true analog or virtual analog synth for those parts.
At the high end cost wise I would definately go with
http://www.davesmithinstruments.com/products/p8pe/ for a true polyphonic analog synth.
If cost is a big factor then I would go with a good virtual analog synth like the Roland SHO1 Gaia
http://www.zzounds.com/item--ROLSH01 or the Novation UltraNova
http://www.musiciansfriend.com/keyboards-midi/novation-ultranova-synthesizer. Though the only drawback is the 37 key keyboard.
And the Atari ST definately would not have made any of those synth sounds. It was actually incapable of making any good sound apart from simple beeps. All music created on the ST was done using MIDI with external keyboards ( as the ST was the first computer to have MIDI built in ). At the time the Commodore Amiga was much more capable of creating onboard sounds. But even that wouldn't have been suitable for music performances.