Unpressurized flight in jet aircraft is really no problem. The biggest consideration is the higher fuel burn at lower altitude.
I ferried a 727 from California to OKC once that we could not pressurize, we used the O2 masks and flew back starting at FL27.0. About an hour out of OKC we ran out of O2 and descended to 9,000 feet for the rest of the flight. We climbed out at the normal rate of climb, but on descent I keep the rate of descent to around 1,500 feet per minute just for comfort.
Of course there was no passengers and it got pretty cold. We did have some bleed air coming into the cockpit, but not very much and the outflow valve was wired open. We had a multitude of probelms with the entire pressurization system, the outflow valve being just one of them.