I am able to understand できるもんなら作る, but I don't understand the way ってのが is used, which I've come to learn is the contraction of というのが
So, in which way is ってのが being used?
っての is というの, and が is a plain subject marker. So it's basically like this:
"I'd make it if it were something I can make" is ...
This is an incomplete sentence, and the corresponding predicate is missing. Maybe you've missed the remaining part of this sentence in the following page, or maybe the predicate of this sentence can be inferred from the context. If this still doesn't make sense, please share the context (perhaps several sentences before and after this).