Portable Air tank repair

Son in law stripped the threads on the cut off valve, and did something to the outflow valve when he filled my portable air tank last. It still has 50 lbs of pressure in the tank, but it won't fill tires when applied. I can buy replacement hose, valves and pressure gauge, but I don't know how to attach them or take off the old ones. Of course, I know I have to empty the tank of all air before I attempt any repairs at all. That can be accomplished with a tire gauge. I'm upset, though, that I can't use the tank because I bought it mainly so I would have access to air to keep my mower, trailer, and wheelbarrow tires pumped up. As it is, they are all slack, and the wheelbarrow tire is so slack it has completely fallen off the rim. I'll have to remove it and take it to a tire shop to be fixed. Strongly tempted to buy inner tubes for the barrow and the cart. Had to put inner tubes in the mower tires because I had problems with them all the time getting off the rim and going flat. They sure don't make stuff like they use to do they? What junk we are having to buy these days. Wish to heck the government would get a clue and put our people back to work so we could have access to some quality merchandise again. Any advice? Maybe I should just write it off and invest in a real tank.