You know what? I'm going to say Po stomps Captain America now that I think about it. All Po would have to do is get around that shield. I would post feats but this is a school computer and I can't post vids from you tube.
Po should actually take this one easily, he's got crazy skills and isn't affected by normal rules like Cap is. As others have said, Po knows a few moves that could easily KO Cap, not to the death since neither is bloodlusted.