Keith........a non-running 9N with NO OIL pressure, is worth about $500, running with OIL PRESS is worth about $1500. While PRIMING maybe necessary, I'd first change yer OIL PRESS GAUGE. ($10, cheap)
To prime, remove the BIG NUT behind the water pump. CAREFUL, It is SPRING LOADED. And yes, there is supposed to be a flat spot on the pressure relief piston. Pour about 1-cup of HEAVY oil down the hole. I use 90-wt outboard motor gearbox oil.
While normal oil pressure is spring relived at about 35-psi, many well worn engines can't make 20-psi and live quite well on low oil pressure. Unfortunately, the ONLY CURE for low oil pressure is an engine OVERHAUL. (parts $450, machine shop $1000)
Besides the issue of rusted sheetmetal repair; new paint job, $500. Rattlecan paint job $100. (tip: ALL 9N's came from the factory, GRAY) New ag-bar rear tires, $250-ea; Sherman aux tranny, $500-700.
So you see, it is eazy to squander $2000 to make a $1500 3-speed 9N sell. .......HTH, Dell