I have a question that hopefully someone can answer and that is what do I use to make pieces that I don't have like I want to add a mask to my guy but I don't have any pieces or things from other guys that I can use and I would like to figure out what to use to make custom pieces.
You could use epoxy, clay or something similar to sculpt parts
It's even possible to create a mold of the parts you sculpt (or existing parts) so you can duplicate them.
You could also cut up some plastic bottles or other plastic containers.
Using a hairdryer or heat gun you can shape the plastic how you want.
There are many other ways but I hope these point you in the right direction.