Hi Dajt,
Okay, cast iron it is! Sometime in the future, if you have to remake it, you might consider bronze, gun metal, or steel. Cast iron is pretty brittle. You can always make slides for the cross head out of cast iron if you feel it is necessary.
Yes, I got my Super Simplex running on air in 9 months! I started in June of 2015 and the first test on air was in February of this year, 2016. I am now deep into boiler construction. I am an experienced machinist, but I know nothing about boiler building. The boiler has been a real learning experience for me! I hope to hydro test before the end of December.
David