Reading your symptoms carefully, I picked up on your comment about it cranking but no glow plug light like usual - can you exaggerate on that? Coupled with the sputtering/no fuel makes me wonder if you have an electrical problem. In this weather, you will need glow plugs unless your garage is very warm. Even if your garage is at 70F, your engine may still need glow plugs to properly start.
Bob asked if your fuel solenoid was opening completely and I did not see that you answered that. Have you checked that? When the engine cranks, are you getting any exhaust smoke?
If you did not open the fuel system but the system appears down in the fuel filter bowl- you may not be getting fuel from the tank. Again, possible electrical issue.
I would go back over all the changes you made since it last ran and see what changed - disconnected wire, blown fuse, wire burned?
If you have the manual, troubleshoot why your glow plugs lights are off. You can download a manual if you don’t have one. If your engine is cold, it likely won’t fire without the glow plugs warming the chambers even if it’s getting fuel.
And as advised before, reconnect the seat safety when you reinstall the seat pan and new seat.