I would just call it a converted Saiga, and give the caliber. That should tell potential buyers what they need to know, and it won't look like you are trying to deceive somebody by calling it an AK.
Just give a good listing of the upgrades that you did. I would hold onto the original parts and throw those into the deal as well.
I partially disagree. I wouldn't care what was on the original FA-10, as that is NOT my problem (and I refuse to make it such). [I specifically stated it this way since I've seen people write wrong info on FA-10s and feel no need to "continue the error" if one were to exist.]
I would put down whatever model number is etched into the side of the gun itself.