1950s-60s "Standard" flush valve

I worked today on a "Standard" toilet that I think is 1950s-60s vintage.

The flush valve resembles and operates very much like a Mansfield.

Does any one know if the flush valve is repairable.

This is the first one I have ever run into and I have a feeling there are no repair parts avaliable.

anyone familiar with this

You might try this link from
Plumbing Parts Depot
http://www.plumbingpartsdepot.com/