It is only 7 days so if i do i'll need to focus on the game during these days and having the engine ready would help (the rules seem to allow that).
I get you, If I enter the jam I'll certainly use my engine as well, I think when you use your own specific engine the line is close between the fixed game systems you will always use because you're not interested in doing anything else on these fronts and the engine itself.
For To Hell With Guns we spent the week creating 95% of the content of the game, all the assets, the maps, all the monsters, 90% of the weapons, but sure for the weapons for example we used some preexisting code, and spinners and such already existed, we just put those on a map, the combat system we just used the one already in place, the stats were arranged in a particular way but what skills actually do in practice was already implemented as well.
This RPG jam was great because I finally dared to share something, and it was important to keep me motivated. Fun fact about To Hell With Guns, the game was rated so hilariously bad during the jam that the game has no sound but it was rated higher in sound than in gameplay, not a big deal but what kept me motivated was rather the comments on the Codex about the game.
I think it's both fair in the sense that we're making a novel game which is just the same kind of games we make, and I think it's nearly what someone who'd pick RPG Maker would do, but also totally unfair compared to someone who simply opens Unity and starts to code. It does not matter that it's a competition only in name and in fact only an opportunity to make games, it's just all the more fun if everybody's using the same rules either way. So yes, in the case I join the jam I will use my engine and plenty of already implemented rules but I understand that the circumstances are not the same for everybody.
