Check the service manual, It shouldn't be that hard to put back on, You may have to start from a far off angle to align the pegs with the holes on the chassis and drop the piece down, then re-attach the spring to its hook on the mechanism's chassis so the piece would lock on the spindle or be close to it depends on the design.
If I would have to guess, it should go as illustrated in the attached picture, You can make a hole in the white piece and add a small hook to it to re-attach the spring end to it.