I’m not familiar with any VN engines (I have never made one, but the interest in doing so has sparked from time to time), but I’m familiar with Godot. Am I allowed to participate in the jam if I spent what’s left of February making a VN-making template and then start actually making the novel once the jam starts?

I think that would be fine. I'm excited to see what you make!

Edit: Community consensus at this point is that premade assets only count as premade assets if they are publicly available (behind a paywall is fine). I know it's late to share this, so I won't be too picky about it, but it will be outlined more clearly in the rules for future jams.