Depending on how the EFX are routed, you may be able to get a pretty good guitar sound.
Are you looking for lead guitar sounds or rhythm guitar sounds?
Lead sounds are a bit different to program than rhythm sounds.
For a good lead sound, I would take a look at the Insert EFX and see if you have a Tube Amp emulation or distortion emulation in there. I would use those in the Insert EFX and leave the master EFX for Reverb/Delay/Chorus for the final processing of the guitar sound.
Another thing to consider is that you have think like a guitar player to play a guitar sound on a keyboard. If you are doing a lead solo, you need to make sure that you are liberally using the vibrato (mod wheel) a lot, because that is what most guitar players do. Articulating the sound is just as important as the sound itself...
I hope that this helps a bit. I do not have an MO6, so I am not aware of how their EFX system is configured.
Grace,
Harry