Would be a fun fight. BJ had very good boxing, but imo he isn't winning the stand-up against Max, who's bigger & taller than him despite fighting at a lower weight, and is deadly from mid-range. Max has filled out his frame and is now strong enough to clinch & grapple with Lamas, but he'd be out-skilled on the ground vs BJ.
So to answer - If Penn took him down like he did with Fitch, he'd need to finish to win. If it stays on the feet or Max escapes to his feet Max wins on points. Very hard to finish BJ in a stand-up fight.