Looking at the Helms Manual, the input shaft bearing at the front of the transmission fits into the case housing. The sleeve that extends outward where the throwout bearing rides appears to be cast as a part of the case. The main shaft bearing and oil seal is captured in the case and held in place by other gears and retainers on the internal part of the input shaft. There is a large bearing retainer plate at the rear of the trans, but I don't think that's the one you're referring to. There are no parts listed as 'retainer' at the front end of the shaft in the bearing area.
Bearing retainer is definately not replaceable. Finally got the throw out bearing off today. Retainer might live, after all. Does anyone have an accurate length of the retainer from the flat surface on case to end where input shaft protrudes?