Here [1] is another interesting low-cost version of ventilator. It's an immersion two-phase continuous positive airway pressure (CPAP) device, of which all the parts are printable. The patient still needs to be able to initiate breaths themselves, but it will cost a lot less energy to breath.
Combine it with a (admittedly lot harder to make) homemade pressure swing adsorbtion system that delivers high-oxygen air, using zeolite sieves and alternating pressure [2].