I’m not sure I know what you mean by ‘bellows’, but the MIT design has, at it’s core, a BVM (Bag, Valve, Mask) device, which is used in emergency situations to help a patient breath while the medical person squeezes the bag with their hands, carefully, and monitoring the person to get the right frequency and amount of air. These can be used for emergencies, but are hard to sustain. The idea of the MIT project and others is to automate the squeezing of this bag with a motor and control electronics, as well as a pressure valve to insure the machine doesn’t push too hard. Hope that helps answer your question.
BTW, here is a BVM on Amazon: https://www.amazon.com/Training-Valve-Mask-Infant-Mesh/dp/B086679TPC/ref=sr_1_6?dchild=1&keywords=bvm&qid=1586110181&sr=8-6
Not sure if that is the exact one they will specify, but at least I think it’s a good start for prototyping. Also note that that one is for infants only (same device, I believe, but a smaller mask for small kids). Last week I was able to order one on Amazon with two masks, an adult and an infant, but those seem to be scarce on Amazon today. Hopefully the shortage is temporary.