Actually, if you want to get technical... the oil pressure gauge isn't original. Originally, it never had one to begin with.... only an idiot light in the gauge cluster. As far as I'm concerned that's as useful, if not more useful than the gauge.
I'd also say... if you've never seen diesel mixed with engine oil... it's not really something you need wonder about. I had it happen a year ago on a Detroit 3-53 and in two hours it dumped the better part of a tank of fuel into the base. I think we drained off 2-3 buckets on the change. Oil pressure was nil at idle and mabey 20 at full throttle. So if you got fuel in the bottom, you'll know it. Quickly.
PS pumps were very common failures on those tractors. That's the more likely scenario. If you get it before it buggers the pump you may get away with changing the shaft seal. If the bushings are bad, you need a pump.