Use whatever Germanium you like. In that circuit, it's not going to make a whole lotta difference. You can sub schottky, like BAT41, BAT42, BAT46, 1N5817... Or install a jumper. Seriously, that will work too. Install a socket and experiment. Do not pay more than a buck or two for a germanium diode and avoid eBay. There are good parts on eBay, but I think the bogus ones outnumber the good ones.