In my shelter-garage conversion, I have taken a magneto from an exercise treadmill, mated a bicycle > tire against the axle, wired that to an inverter which powers a battery charger to recharge my batteries.
The batteries are then wired to an inverter to power my squirrel cage fan to draw in fresh air through my filter system. The shelter is sealed so a positive pressure system exists inside the shelter.
The CO2 removal is accomplished by a 2" pipe mounted 1/4" above the floor; the pipe runs outside where the exiting CO2 pushes open a rubber flapper valve which closes when nothing is being pushed out. I have a couple of vacuum cleaners (old ones) that act as back up pumps in the event my squirrel cage motor fails.
I have the materials on hand to make a hand-powered piston in a cylinder to draw air in and push air into the shelter. I use axel grease to both reduce friction and help reduce air loss while operating the pump. Rubber flapper valves allow air into the cylinder and keeps air from being pushed outside versus inside the shelter.