@ixtility @VeeRat There were folks doing #PAPR rebuilds on obsolete designs and auctioned government surplus. During the ripening phases of the let-‘er-rip period in the US, hospitals were known to surplus whole sets of unopened PAPR units as soon as a component expired, even if that component was a single-use emergency long life battery.
The 3M BreatheEasy could be shockingly cheap since it was obsoleted by the company. It could be made to run off USB power banks matching its current with some basic soldering.
That Sentinel XL you linked was popular since it has a replacement part option that lets it take civilian batteries instead of its proprietary rechargeable unit. It also has standardized 40mm NATO air connections which made it compatible with lots of other surplus gear and current-production gasmask filters.
There’s no centralized guide about this that I know of, but the project seems very doable. Please take all possible precautions if taking your life in your hands with this knowledge, IANAD/L/professional, this is not any kind of advice and will not mollify the North Carolinian rules lawyering dragon once the fell beast is roused