I think you might use some non machineable cast rod, and it would be cheaper than N99.

But welding cast is tricky and not for the average welder, particularly in a serious structural situation like this, where something can be life or death. To weld properly, you'll literally have to set the thing on fire, getting it hot enough to destroy all the internal seals, and cook the oil into sludge...then cool it slowly over a long period of time.