G151? Now that's rare!
The Gemini and early Rodeo/Jackaroo SOHCs are the Z series (G161Z, G180Z & G200Z).
These are very different to the G161S & G180S SOHC that were in the Belletts etc. (Bearings, conrods etc are mostly the same as with many Isuzus G & 4Z series engines, but the Z's are the Opel-ised Isuzu engines AFAIK.)
The S-series looks like the old SOHC Datsuns; they have the inspection hatch on the side like the twin-carb version shown at
http://www.n-classiccar-jp.com/car/isuzu/070519bellete/070519bellet.htm above.
And although the G161S & G180S look similar, many parts are different sizes (oil filter assembly etc).
The G180S block is the same block (effectively) as the later G200W (and I assume the later blue G180W).
The Bellett at Centre Rd Wreckers (Melbourne) had a G180S. The head is still there, but the block is gone.