erm new to this jam... can we use assets we own? as long as we start now during the jam or does it need to be from scratch?

from the Rules section.

"Your game must be made entirely in the duration of the game jam, although you can use free assets (check Resources section!), assets you legally obtained, artwork/music you’ve previously done, or OCs/stories you developed at your roleplaying tables."

So yeah, you can use assets you own.