The part numbers for the seats will be found in the Illustrated Parts Catalog or Illustrated Parts Bulletin. However, neither this nor the maintenance manual will tell you if you are legal to operate without the seat in place. If the seats are different part numbers but otherwise identical, you may be able to swap seats. With some seat assemblies this is possible, others it is not...in some cases because of the seat design, others the belt mounting attach points, in other cases the shape of the cockpit precludes swapping the seats because of the taper or clearance around circuit breaker panels, cockpit sidewalls, etc. Seats which incorporate track-specific locking mechanisms which may have been mandated by an airworthiness directive may not be swapped, as this invalidates not only the actions of the airworthiness directive (mandatory), but also invalidates the airworthiness certificate in accordance with line 6 of that certificate.
I know this sounds complicated for a seat swap...the point is, there may be a lot more involved.
Setting aside other issues, when you do swap the seat, what do you do with the broken one and can you fly without it? What about moving it to the other side, and placarding the broken seat inoperative? Others have touched on the issue of weight and balance, which must be accounted. Some seats incorporate components such as emergency batteries, or cover items such as emergency power supplies, circuit breakers, autopilot computers, etc. Removing the seat makes the system vulnerable and may present a safety of flight risk.
As an example, within the last few days I dealt with an older turbojet aircraft which had components adjacent to the seat removed. This exposed three wiring harnesses which were normally covered. The aircraft was experiencing erratic control issues. The autopilot was turning itself on, engaging itself, and taking control away from the pilot. Numerous other malfunctions were occuring, which were entirely without apparent logic. Yesterday I was able to determine that one of the three wire bundles, when moved slightly, could get the system to activate and release on it's own. Over and over. Breaking apart one of the pin connectors revealed a single wire that as intermittantly sliding in and out of a pin, an causing a host of problems. A number of us brainstormed over that, and were baffled. We wrung out wiring harnesses, chased things, replaced components. Very frustrating. All because the harness was exposed and had been subject to some inadvertant contact by someone at some time. Imagine the same in your aircraft...could be a cannon plug, could be a ciruit breaker...who knows. The point is that there may be things one never considers associated with what seems like such a simple concept.
It's just a seat...but seats unlocking, dropping, or not fitting properly have caused or contributed to fatal crashes.
You may be able to operate with the copilot seat in the captain position, but depending on the circumstances, you may have to put the bad seat in the copilot slot and placard it inoperative.
Limitations against operating with a seat out won't be found in the maintenance manual. These will by necessity represent aircraft limitations, and must be posted in the aircraft flight manual (and may even be required by placards). The load bearing capacity of your floor may preclude leaving that space empty. In some aircraft, the seat represents a very utilitarian purpose, and may be very station-specific (the OV-10, for example, uses otherwise identical seats, but mounts the parachute for the system on opposite sides...the weight of the parachute assembly determines which way the seat goes as it leaves the aircraft which has a critical determination on aircrew safety during an ejection. I understand you're unlikely to eject from your 402, and in all probability you won't have much hassle removing the seat, but it's going to take some checking on your part and the part of your mechanic.
Lacking a MEL, you'releft to fall back on 91.213...in which case you must determine if the equipment is required (a pilot seat at a pilot station certainly is, as part of the basic aircraft structure). If a second seat isn't required due to lack of a second pilot, then you need to refer to the aircraft flight manual to determine any limitations that apply. If you're repositioning the aircraft for repairs, then you should be obtaining a special flight permit ("ferry permit") which will allow repositioning the aircraft for repairs. The seat is part of the interior aircraft structure, and may or may not require a special authorization to be flown with the seat absent. Some aircraft in the basic equipment list cite the seats in or out, and give options, others do not. I don't have any 402 data before me, and can't quote you specifics about the type design or it's limitations...I can only try to highlight the concept that the topic may be broader than one initially realizes. Good luck.