You should be able to. I've heard of both O-A and O-H welding aluminum in modern times. Hydrogen is cheaper, I think, but not quite as hot and harder to see the flame. If you google O-A welding aluminum you should get plenty of links- mainly for sheet metal and small parts though. You'll need a big tank and big tips to weld anything the thickness of a fork leg- the idea is to weld it before the whole thing turns into a blob.