You are right,drain the 2-t tank, you could use a couple of swarf free bolts to bung the pipework.for convenience ,if you prefer ,and leave what isn't in your gearbox there!.Drain the gearbox,remove the pipes from the oil pump.Detach the pump body,the shaft ,with detached seal,is your culprit.Use a sealant to seat the seal.Reassemble the oil pump and pipework,fill the 2-t tank,if drained, there is a bleed screw on the pump,open this and wait until you get a flow .refill the gearbox( new crush washer).Empty the fuel tank,put in a litre,-Imperial or U.S.?- of pre-mix(25-1)and fire it up.