Unprepared for Modeling Test -- Back Out of Interview Process?

Hi all -- I am currently interviewing for a small PE firm (off-cycle) that I am not that enthusiastic about. I have been extremely busy at my IB job recently and haven't really hard time to prep for the modeling exam -- I thought I would be able to study over the weekend but the firm is now fast-tracking and telling me that I must take the test sometime this week.

Although I am not that concerned about the outcome of the interview, I do not want to make a bad impression on the headhunter. From a recruiter's perspective, would it be worse to back out of the interview process now or take the modeling test when I am unprepared (and potentially do badly). I just want to make sure I am looped into future processes with this headhunter, but I do not know which choice they will look less favorably on. Any advice would be appreciated!