I think you're fine (and I really can't wait to see it) I mean, I would just go ahead with it, maybe put together a page explaining the head and the process that you can link to to cite credit for the sculpt when posting pics of it but if you're really worried about it, you could try emailing the company. Send them a picture of the mods, tell them what you want to do(being very clear about the not selling), and ask them if they're okay with that. As for giving away flawed copies to friends, if you really really trust the individuals to not try to sell them later, I think you're fine. Or you could try stamping/carving something inside the head/headcap saying something like "company name sculpt modified and cast by x. scrap head not for resell.
I think the biggest issues here are the passing it off as an original and selling for profit, neither of which you're doing.