When your Jet Gun has reached the end of its limited lifespan, simply rebuild it from spare parts and terrorize your enemies with it again. The parts you’ll need to bring to the workbench to make this are the Jet Engine, Handle, Wires, and Headlight.
The Jet Engine can be found in the tunnel between stops 4 and 5. The Handle is between stops 3 and 4. The Wires can be found in the Power Station, on the ledge as you drop down through the hole in the walkway. Lastly the Headlight is found in the cabin.