Ehhhhhh... if you're not sure whether an original creator of something is okay with you directly reclaiming their work (credit due or not), then it shouldn't be assumed whether it'd be 'rude' to them or not.
It's not about breaking any breaking laws or forum rules; it's all about respect and appreciation for the work someone else did. While some don't mind that others carry on their projects whatsoever, others could be beyond irate and highly offended if they were to find out about it. Regardless of money not being involved, it could be taken as a disrespect to talented people and their work, and if so, in the meantime you'll end up looking like a no-talent thief for stealing it. And those who put all their love, time, and energy into that project earned every right to feel however they want to feel about it. So morally-speaking, if you don't have consent and there's no easy way to contact the creators, you're standing on shaky ground either way.
The best thing to do in these situations is to just leave it go. And if you really want to continue their work, start from scratch and (if you really want to be classy) dedicate your new work to the original person/team somewhere for being your inspiration.