This is a picture from a BMWMOA post of replacing the sight glass, looks like you carefully put a wood srew into the plastic around the red dot, and pull it out, friction fit.
From the BMW MOA Forum postings
drill small hole in center of sight glass - don't worry about any little bits getting inside the crankcase. They will just stay inside the sight glass.
insert wood screw into hole
pull on wood screw with pliers till it comes out
wipe down the hole in the engine
push new sight glass into hole.
place wooden block over sight glass and tap into place with mallet.
Do NOT use any sealant or lubricant when putting the new sight glass in.
BTW - putting silicon or whatever on the outside of the sight glass is not gonna do nuthin' except look funny. If the sight glass decides to depart the premises like it did on Chaz's RS, the silicon is not going to help one bit. There is a tremendous amount of pressure inside the crankcase. The design of the rubber on the outside edge of the sight glass keeps the pressure from pushing the sight glass out (or pulling it in).
insert wood screw into hole
pull on wood screw with pliers till it comes out
wipe down the hole in the engine
push new sight glass into hole.
place wooden block over sight glass and tap into place with mallet.
Do NOT use any sealant or lubricant when putting the new sight glass in.
BTW - putting silicon or whatever on the outside of the sight glass is not gonna do nuthin' except look funny. If the sight glass decides to depart the premises like it did on Chaz's RS, the silicon is not going to help one bit. There is a tremendous amount of pressure inside the crankcase. The design of the rubber on the outside edge of the sight glass keeps the pressure from pushing the sight glass out (or pulling it in).