You can fly single pilot only FAR23 aircraft, not FAR 25.
Exemption have been granted for some airplanes like the Falcon 10 in france with some "single pilot" mods, but generally it is not widely spread.
by the way the C501 flight manual stipulate that the aircraft is Far 23 if it has (among other stuff) an operable Autopilot approach coupled, a boom mike, a xponder ident on the yoke, etc.... check MEL. if not it reverts to Far 25 !! simple no ?