@ash is selling a module which replicates the organ sound, here.
You can connect it via MIDI-din, and use the Line In on your Yamaha PF-85 to amplify the sound.
As for your next piano, I would suggest that you try before you buy. With your real acoustic piano, and your old but worthy Yamaha PF-85, your standards and requirements will be high.
Certainly, I would suggest that you go for something better than what you have now, rather than a sideways move.