Just out of curiosity: if the original project was open source, why did you decide to restart from scratch?
Open source just means the source code is available. It doesn't mean you can legally use it. That is, in fact, the whole point behind the most famous open source license, GPLv3: code is open source, but there are still restrictions on how you can use the code. I don't know about now (most projects I work on are MIT-licensed, these days), but there was rancor around the move from v2 to v3 because v3 was more restricti…
Copyleft licenses like the GPL come with extra guarantees that do not violate the core guarantees of open source software. Instead, they make them stronger. The 'restrictions' GPL imposes essentially boil down to this: "if you use (parts of) GPL software, you must give your users the same freedoms the GPL guarantees." GPLv3 and AGPL closed up loopholes that allowed people to bypass those clauses.