Temporary Covid-19 ventilators – define the problem, then find options

A lot of people here read the post yesterday suggesting a DIY/grass roots effort to create a ventilator.

Having many more ventilators might save a large number of lives if health care systems become overwhelmed, as seems to be the case in Italy.

Many people here rightly argued against the idea of creating a from-scratch ventilator design because of the technical challenge and the fact that getting ventilators built in factories faster is a better idea. Increased production will happen at some point, but Italy teaches us it may not happen quickly enough.

I think a better definition of what such a device has to do to be useful will be valuable near term.

According to the information I'm seeing from Italy, ventilators are in heavy use, but also things like CPAP devices with hoods, bag masks, and even ambu bags - pretty much any device that can help someone with pneumonia breathe, take in oxygen and remove CO2.

So, my question to all the experts and amateurs here is this: What exactly is needed in terms of breathing assistance to help someone with pneumonia survive?

Would a plain CPAP machine be enough?

Would a simple demand regulator set to provide air at a few tens of millibars above ambient pressure help?

Are there any devices in common use which could provide breathing assistance in dire need with some modifications/adjustments?

Certainly the ideal would be to find a way to make sure there are enough ventilators for everyone in the world, but unless the various governments and corporations of the world start reacting faster and better to this pandemic, we're going to need all the help we can get, and if converting or building something can help one more person somewhere in the world survive the virus, it's worth doing.