The Sig/GSG is 80% swappable with GI spec 1911 parts, so you can replace the mainspring housing with a flat one. I had mine swapped out to a flat MSH with the lanyard loop. The retaining pin is placed differently in the design of the Sig/GSG than the GI spec, but it will work. Don't be surprised if some fitting is needed, that is typical of any 1911.
Hope you have as much fun with yours as I have had with mine.