I agree that you should analyze why that passage is so difficult and figure out ways to master it, then practice it!
That said, I had a performance of Mozart K271 last spring. I thought I had all of it, but needless to say, there were some passages that were very difficult, and I do not perform often. I simplified "on the fly" in one place only, and that was because I could tell I would trip as I approached it. That was in one bar where there was a rapid 16th note bass figuration that was very hard to coordinate with the RH--so I blocked the chords in the LH. This was for 2 beats in a transitional passage that was not particularly noticeable with the orchestra also playing. It was not my choice of how to play it, but in the circumstances, it sounded better than a bobble would have.
That's the only justification I can come up with for simplifying. If you have prepared the piece well, but during the performance, find yourself feeling unsure, simplify rather than stop or make a glaring mistake. But don't PLAN a simplification. That's sacrilege!

Teresa