I wanted to Speedrun JJ and saw that Voyager wasn't mentioned on the rules, even though it doesn't appear in the inventory you can still get it in-game (if you purchased it), it's basically an Ecto-1 but worse, I'm in doubt if it's allowed or not.

It wouldn't be allowed, as it is a Event vehicle. I will update the rules to list it, thank you for bringing it to my attention.