Yokosuka wrote: Are you sure of the synth nature of the original tracks? I'm not a specialist but the high sound compression is maybe misleading.
I'm judging this by ear as a lapsed keyboard player, but it sounds like mostly synth listening to the tracks on the OST.
Though, much like Iuchi intends to do for Shenmue III, it sounds like some live recordings are in the tracks too for instruments you can't easily synthesise (Erhu, etc). Listening to the OST and the orchestral version of the main theme, it's pretty easy to tell the OST is synth because a lot of the backing instruments don't have that 'real' sound.
It really depends how many recorded instruments are in the track, but they could always fade them out, have the synth chords transition, then fade back up recorded instruments.