Aftermarket oil pressure

If I wanted to fit an aftermarket oil pressure dial and sensor, how would I go about it? What size is the stock oil pressure switch fitting?

Following, guessing gen. 3?

Suzuki's M-series engines use a 1/8" BSPT fitting - please use a flex line from the block to a hard mounted tee and run the gauge off one side of the tee and the oil pressure switch off of the other.

thank you!!! i had a lot of trouble finding out the thread of the oil sensor, i'll do as you say!

Car and Motorcycle manufacturer’s stopped fitting oil pressure gauges to vehicles 50 years ago because modern engines when hot and at tick over have a relatively low oil pressure and it was considered too worrying to the user.

I am not sure what you hope to learn from oil pressure, but suggest that oil temperature might be more useful, at least it will show you if you need an oil cooler for prolonged high speed driving.

This doesn't apply to the gen3, but the gen4 Jimny I believe has a variable displacement oil pump, which means a gauge won't actually tell you a whole lot as the pump will change flow rate depending on RPM and oil pressure - if the clearances in the bearings increase it will still try to maintain the "correct" oil pressure. 

In a gen3 though, go for you life, although it probably will just make you worry more and won't actually tell you that you've run out of oil any sooner than the stock oil warning light will. 
It is fun to have a row of gauges on the dash though