As Dusty said, no compensator but other features not found on a pre-1919 pistol are allowed in the Modern category. Consult the Handbook for the specifics.
I shoot in the Modern category; my pistol is a Series 70 Gold Cup with several upgrades that are fine, such as an extended thumb safety, beavertail grip safety, Commander style hammer, match trigger, checkered mainspring housing, ramp front sight, beveled magazine well and a full length guide rod.
On the other hand, I had to remove the exterior mag well funnel and the add-on wrap around checkered panel for the front strap.