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Those retainers are located to put reverse gear in an exact location.----You need to take it all apart to inspect all parts for rust damage.----If all is well find a good used case to make this repair.
The part in the back is called a bearing housing and when installed it does NOT rotate.------Used gear housings can be found if you look.-------But you have to inspect all internal bits first as they may all be rusted beyond repair.-----This is precision stuff so YOU must inspect it!!