There should be pipe plugs around that oil filter.
I would install the oil gauge there for the time being to see if there's oil pressure, you may need a pipe reducing bushing in order to install the gauge. Most of the gauges are 1/8 pipe threads. We used a lot of gauges at work when I worked for the US government. We tested all the
vehicles for the US Army. Some were the M1A1 Abrams Tank, Bradley Fighting Vehicle, M60 Tank
and we ran drawbar pull and cooling tests on just about everything. I was even drafted in 1953 and was rehired after I was discharged. Hal