I can do a fair rendition of "Smoke" on keys, especially the solo, but... the guitar parts are guitar parts, and to me they'll always sound better on a real guitar. Still, there are plenty of juicy organ parts from "Machine head" ( the album that spawned "Smoke", and one of the three very first albums I ever bought with money that I earned myself.. Brain salad Surgery ( ELP ) and Led Zeppelin ( 4 ) were the others.
I may be able to fool the average listener on a recording though. As an example of this I gave a friend who has played professionally as a drummer a couple of my songs to listen to. He knows I'm not the most proficient guitar player, so he asked who I hired to do the guitar parts ( I can play a little, but not to the level of the intricate solos on the recordings ). He was very surprised when I told him that I had done them myself on my Motif / PA80.
He had also stated that he disliked drum machines ( he was probably thinking of the older ones that sounded very mechanized ), and was very surprised at what I was able to achieve using software and / or real time playing on a keyboard.
Dark Side of the Moon is another story. Lots of synth stuff going on there. It's mainly a matter of emulating the synth sounds and setups that Pink Floyd used. Although there are plenty of guitar parts on that album, they don't always take the forefront, and if one worked at it, you could make a fairly close reproduction on a good synth. I was able to do it on the song " Shine on you crazy diamond " .. a PF favorite of mine from a different album.