Run by just blowing air into it. Demonstration:

Exploded view in 3d viewer:
https://skfb.ly/6LQBR

There's a LEGO compatible version of the motor:
https://www.youmagine.com/designs/air-balloon-motor-for-lego

If you like how it works, you can support me by purchasing the Pneumobile chassis:
https://cults3d.com/en/3d-model/game/pneumobile-pneumatic-toy-car (3d Viewer: https://skfb.ly/6LV6D)

Extra parts needed:
- M3x8 Bolts (x15)
- M3 Nuts (x15)
- Thin Nitrile Gloves (thumbs from M sized gloves worked the best)
or Silicon (worse) or PET (not tested) ones

Optional parts:
- M3x12 or longer bolts (2x)(for the gears)
- M3 Nuts (x2)(for the gears)
- 6x4mm Silicon Pipe (can be found in aquarium shops)
- M5x10mm Washers (x2) (instead of printed ones)
- 105ZZ Bearings (x2) (noticed no difference in performance)

Parts with "X_" in their names are optional.

Assembly:
- Use 3d diagram as a guide: https://skfb.ly/6LQBR
- Cut off a glove finger tip (thumbs from M sized glove worked well)
- Pull it onto a Cylinder_Ring (Use Helper_Finger to leave enough height)
- Connect a Piston to it with two Washers
- Clamp the Cylinder_Ring between Cylinder_Plate and and Cylinder_Lid
- Clip them with Helper_Clamp so they won't fall apart at once
- Bolt them together to Plate_Top
- Repeat for the other 2 cylinders
- Insert th Crank with 2 Washers
- Fix it in place with Frame_Bottom
- Bolt all the parts together
- Put the Valve_Inner in place
- Bolt the Valve_Lid into place
- The engine should be running!

Troubleshooting:
1. Rotated freely by hand, but struggling when working - the inner valve may have too much friction against the frame (sanding the valve and the frame may help for a while).
2. Jams on a certain cylinder - this glove finger may need a bit more slack.
3. Isn't rotated freely by hand - gears bolted too tightly to frame (should have a bit of play), some gear teeth didn't print very well, or the whole gear printed with "elephant foot" and runs too tight with another gear.