Remington 1100 12ga 3 inch Mag barrels only have one small gas port to allow the correct amount for Heavy loads which is 12ga 3 3/4dr 1 1/4 or larger payloads as in 3 inch Magnum shells.
1100 12ga 2 3/4 inch barrels have two gas ports and of larger diameter for lighter loads.
If you want to shoot 1 1/8 ounce loads in a magnum you have several ways to handle it.
(1) If shooting singles, no problem, you have the best of all shell catchers, manually extracting the case into your hand.
But if you want the gun to cycle then you have to do something else.
(2) Use a higher power load will often work, like use 1300 fps sporting or handicap loads. They will work the action hard enough on many magnums to cycle.
(3) If you hand load, use a slower burning powder and more of it, that moves the pressure curve further down the barrel, and you probably can find something that works. Like if you normally shoot maybe 18 grains of red dot, you might get the same velocity using a bit more of green dot, like 19.5 grains, and this might make it function, it does in my magnum 1100.
(4) You could buy another barrel (2 3/4 chamber) and this is the most common solution people use.
(5) You could drill your gas port larger or drill a 2nd gas port. This has also been done pretty often and works but then you should not use full 3 inch magnums in it.
Changing the action bar and collar is not your solution. It probably still won't cycle if it did not before. Also the magnum action bar is more desirable in my opinion, because it gives less recoil than the standard.