That Korg, and MANY Korg synths fall VERY short in the acoustic piano dept.
I doubt that you will ever find a suitable acoustic sound with that keyboard. How about an add on module? There are TONS of thise around. I have a Roland SC33 you can buy that will give you 28 more voices, and a whole new GM/GS set of voices. WONderful classical pianos & strings. Lots of variations. It's like adding a complete Roland keyboard .... only WITH OUT the extra keys !