How hard can this be? You already have power beyond available at the diverter valve, but should you choose not to go there and fabricate your plate instead, you can identify the ports you mentioned easily enough. All you need is a rubber tipped air nozzle and compressed air. Fully lower the lift arms. Remove said plate and expose the ports. If you bump the starter, oil should flow from the pump's supply port. If you introduce air pressure to this port, it should go nowhere. Remove the filler cap and move the lift control lever to about midway or so. Then introduce air pressure to one of the other ports. With one(sump return) air will be heard escaping through the filler opening. With the other(pressure in), the lift arms will rise. Now you know which is which.
