Now load up your MIDI file, assign OmegaGMGS2 as your synth, and listen to the past sound better than you remember it.
Software like OpenMPT or Renoise loads SoundFonts directly. OmegaGMGS2 provides "fake" real instruments that sit well alongside SID chip or NES waves.
Look for the latest version (typically v2.3 or higher).