Even steel (welded or formed) air tanks have a compression (crush) idea behind them, due to the danger of compressed air explosion... that's why they're always a seam-welded tube with end caps that are nearly spherical... the entire weld won't fail, and the air will rush out before it can explode.
Lots of people make compressed air lawnmowers, it's quite easy, if you can get someone to weld the cam for you... the real kicker is that you can make more or less power and use more or less air based on the cam profile you decide to grind into the new cam.
Also, with a single cylinder air engine, it completes it's power cycle EVERY revolution, rather than every other. (intake and power are the same cycle, there is no compression, and exhaust is obvious.) So it essentially becomes a two cycle engine.
Ever wonder what to do w/ that old Two cycle weedeater you don't use anymore? Got an old plastic water tank from an RV laying around? 40 PSI in a 10 gallon tank strapped to your back will eat your weeds for free! :P
You can also get air to fill a small tank or a tire free at some gas stations.. (most are 25-75 cents though, for 3 mins.. enough to fill at least 2 10 gallon steel tanks to 120 PSI (max))
So I hope that gave you something of a starter to work with, but the information is readily available, you just have to peek around the right corner
Happy searching! Oh, and welcome to EcoModder!