Hi Geoff.
I purchased my vent valves from a company called "Tanks". They are in Clearwater MN, USA Phone 320-558-6882. They are aluminum remote mount and cost $11.0 each. They are about 1 inch in diameter at the vent end and have a 3/8 barb for a rubber tube. I decided to mount the valve part as near to the tank as possible,( probably right on top of the vent tube of the gas tank). In the event of a rollover you do not want a tube full of gas that could catch fire. The valve has a 1/4 NPT male thread on it, so a female NPT to 3/8 hose barb connecor can be connected to it to facilitate routing a vent tube to where ever you want the actual vent to be. The vent is 1 inch diameter and has a 1/4 NPT female thread. A male 1/4 NPT to 3/8 hose barb connector will fit into it. I have not mounted my valves vents yet, as I am still deciding where to put them. I am reluctant to put them near the filler caps, as rain water could get into the vent tube and then into the gas tank.
Mike