Some people will say that a 34 is fine and others would say you could get a 36 and UPenn would still be a longshot for you (or anyone).
Both are true statements.
I think there is value in attending a highly selective institution. The admissions process for a school like UPenn seems to be 50% mysterious and 50% rigorous. I think you (or anyone) would need to maximize every data point to have the best chance. Would a higher English score make a difference? None of us know for sure, but it is possible it could matter, and you will only do this once, so take a shot at improving it.
Because of the importance of the English and math sections, consider regarding your composite score as perhaps a 33. Then decide whether to retake the exam on this basis.
Were most of your questions you got wrong in the grammar part? If so, do some review and retake. My kid did 2 hours of review (with a tutor) and her English went from a 29 to a 36.
UPenn superscores, so definently try again. With overall being 34, getting english to 35 or 36 (which is the easiest section to improve in) will likely bring composite to 36.
Math and English are the two easiest sections to improve on on the ACT. English is just grammar rules and many are weak in this area. Easy to study thought. Good luck!