Were the crack only involving the stay I would say repair is not unreasonable, however it looks to me like the dropout is what has cracked. This is a larger job, and I suspect there will be opinions ranging from just JB Welding it to needing a new frame.

If this was my bike I would explore a repair but only because I can do my own labor. If this was a customer's bike I'd suggest moving on. Undoubtedly there's other stuff this frame could be bothered by, like nearly rusted through seat tube at the BB shell or the RH chainstay also at the shell. I don't think I could suggest a repair without more info and perhaps some initial disassembly to see inside better. Andy