yeah you gotta hog it out from solid round. guys on the small engine forum 'smokestak' use just regular cast iron pipe for cylinder liners all the time but those are low rpm low stress applications, that is more of a grey iron than the ductile you would probably want.
then again, take something like a india-made Hero 65cc kit... what are the chances their metallurgy is realy that good? ductile goes through a pretty crazy heat treating process i doubt anyone is replicating for cheapo moped cylinders, so who knows.
if you call up durabar in chicago and talk to their application engineering folks, they'll send you a short engineering sample, but yeah, you gotta hog it out from solid. Durabar is good stuff though, cuts way better than steel but it has similar properties for fatigue and ductility.
There is a good video on youtube of a factory making two stroke cylinders, showing the casting process... i think it might be the polini factory or something?
One thing they do, similar to the way they cast engine blocks, is they make the entire base gasket surface a giant riser, basically extrude the whole base gasket face 2" up to the bottom of the skirt, feed it from a sprue into the 'top' of the cylinder which is upside down, and then that whole base gasket surface keeps pressure on the mold. Dont be afraid of making a giant riser, the other nice thing about it is that it keeps the whole casting hot so it cools slower and you get less warpy stuff, and it gives you a lot more flexibility if you run short of material to avoid a short fill, plus with a lost wax dealio, it will burn out better.
cylinder fins are notoriously difficult to cast, most air cooled cylinders are done in permanent mold exterior with no bake or plaster cores, the permanent molds can be heated up real hot so they flow. you want to be about 400 degrees to keep from getting blisters and weird surface finish problems.
I'm super stoked on your progress, this is huge! I'm getting my printer tomorrow and cant wait to get going on catching up to you, haha! I've got a long way to go on the printing front.