Hello. I think it's okay to use assets, sources and other packages you already prepared.
Bastiaan gave this answer on Discord:
The whole idea around the jam rules is that you don't bring a half finished projects into a jam and get a headstart on everyone else. Thats why a theme is involved and not announced until the start of the Jam. If you use some assets you've got lying around, I see no difference to that compared to a jammer who downloads an asset pack from kenney.nl for their entry
If you started a week earlier, to prepare your project, I think it's fine. You can also say it in your entry description, so everything is transparent.