These seem contradictory since the existing wiki is little besides plagiarism.

Even just the front page contains a Did You Know section that consists mostly if not solely of verbatim text from the J2O Did You Know box at the bottom of its front page. See here for a captured list of J2O Did You Know facts from some point in time.

Having clicked the random page link a few times I got (without omitting any results besides duplicates):

• Medivo, a verbatim copy of the J2O article on Medivo;
• Jazz Jackrabbit: Holiday Hare 95. It's a picture.
• Facts about Jazz Jackrabbit 2 (programming), which was apparently written by Sfaizst. It's all over the place, with broken English, and pretty bad article material overall, but that sounds like Sfaizst, and thus probably original content;
• Jazz Jackrabbit 2 Development Versions, a copy of Jazz Jackrabbit 2 Beta Versions with some minor rewording;
• Chuck, not a lot of content but possibly original;
• Orbitus, again a verbatim copy of the corresponding J2O article;
• Deckstar, another JJ1 planet so no surprise here. J2O article;
• Jazz Jackrabbit (GBA), probably original, although quite underwhelming in comparison to the corresponding J2O article;
• Stonar. JJ1 planet, you know the drill;
• Lori Jackrabbit; contains some verbatim text from Lori Central but seems mostly original. Starts with an unsourced claim that Lori is the youngest sibling, which I'm not convinced is possible to confirm (outside of the sense that she was the last one created);
• Jazz Jackrabbit versions, excellent article, 10/10, but verbatim copy of my social life;
• Exoticus, what you'd expect.
• Marbelara, am I just really unlucky or does this wiki have little besides JJ1 planets?
• Sluggion. I think this is a good time to stop.
• SlavikPlay.

I hold a firm belief that the Wikia wiki was originally started with the intent to copy all J2O wiki content (including the background, as noted by Stijn). Presumably eventually some unaware editors joined and started adding original content.

This is not an attack aimed against you, or Wikia per se, but do keep in mind that if you wish to have a plagiarism-free wiki, you will have to purge most of the articles currently on there, leaving you with, if the sample above is of any indication, maybe 20 articles with any content in total. It's only slightly short of creating a wiki from scratch.

That said, I imagine it's better to have someone who appears relatively responsible in charge of it than the 10-year-old kid who I believe originally founded it, so good luck to you.