Thanks to Charlie Mullendore of Antietam Classic Cycle for sending me this information via email. In Charlie's on words:
I made my own from PVC pipe and fittings. A length of pipe, two caps and four brass barbed fittings (3 for 5⁄16 inch hose, 1 for 3⁄8 inch). Inside is a stainless steel scrubbing pad to act as an oil separator. The forward fittings (5⁄16 inch) go to the rocker covers, the rear ones go to the oil return to the sump (the 5⁄16 inch one pointed down) and a hose venting to the atmosphere (3⁄8 inch). A piece of this rubber and a hose clamp attaches it to the frame.
Engine breather made from PVC pipe; fit to Moto Guzzi small block models.