Ethan,
First thing I would check is all the safeties if they are all connected, with the switches, or check any jumpers where the switches should be, if they are jumpered.
Factory safety's are: Brake pedal applied to start, and PTO in the OFF position. The electric PTO switch is actually a 2 position on switch. When the switch is down, in the "PTO - OFF" position, there is a circuit that allows the starter to function. When the switch is up, in the "PTO - ON" position, there is a circuit that allows the PTO to operate. These positions are mutually exclusive. If the circuit is made for the starter, the PTO won't work, if the circuit is made for the PTO, the starter won't work. There are times on mine, if it has been damp, or been "sleeping" for a while, I've had to wiggle the PTO switch while it's in the off position to get the tractor to start.
There is a brake switch under the tractor on the rockshaft the brake pedal operates. This switch can fail, or there is a "flapper" that pushes the switch when the brake is depressed. If either of those have failed, no start.... If you have the tunnel cover off, you should be able to see the brake switch........
Electrical on the Quietlines is fairly simple. If you need some direction, ask away, as I just recently fixed all the electrical issues on a 1650. A previous "mechanic" (not the owner) fancied themselves an 'lectrician.......