Happy to report:
Followed cranky crank's excellent advice and removed and refitted the muffler, bonnet and radiator, 3 x manifolds and entire front pair of cylinders, head push rods & rocker gear and replaced all 4 x 2" and 4 x 1.5" concave (not cup) core plugs in the block and head.
They are a bit of a trick to install. There was a very accurate/informative video on a similar Minneapolis Moline twin city prototype tractor restoration on Youtube. We meticulously cleaned their stepped cavity paying attention to the inner 90 deg radius, applied Permatex aviation form a gasket to the plugs for insurance and belted them with a big drift until they were slightly ~ 1mm indented like in the video. They expand in the hole. While the cylinders were off we flap wheel die grinded a small top ring lip off the bores. For re-assembly we put it in gear with one piston all the way up. Encompassed that piston with an improvised tin/hose clamp oiled ring compressor and securely suspended the cylinder block over the long studs coming out of the crank case and gradually lowered it over that piston. (Yes cranky there is a 45 deg radius on the bottom edge of the cylinders) Once that pistons rings were well inside the cylinder we removed the compressor to the other piston, put the tractor out of gear and gradually turned the crank lowering cylinders with the inserted piston at the same time and the ring compressed second piston rising into it's cylinder. It literally all slid back together very nicely. We had an owners manual that emphasizes that you attach and torque the exhaust/intake, top water and left side water manifolds before torquing the head to avoid gasket later leaks. Different sources quote cylinder head torque specs around the 95 - 100'lb. Surprisingly the only gasket we needed/cut was the block to crank case. The cylinder exhaust/intake manifold and head gasket are laminate copper, paper, copper type that cleaned up nicely with thinners. We sprayed the head gasket with Hylomar blue compound, used high temp silicone on the exhaust manifold and lubed all threads with copper coat.
Once it was all assembled we fire hose high pressure flushed from the cylinder head manifold, through the engine and out through the water pump and removed an considerable quantity of shale. This also tested the retention of the core plugs - no issues
We replaced the belts and water hoses and clamps. The top radiator hose cast flange had a rust growth and warped where it met the radiator. We filed it flat and butane torch sweated the brass riveted inner and outer top tank plates off. The plates solder had degraded and come away from the tank. We made a 2 x half inner flange plates to get inside through the hole with 4 x tapped threads. Purchased countersunk head stainless bolts, made a gasket and fastened our new inner and outer flanges to the top tank with sealant. Surprisingly a Nissan all model Skyline 78 deg c stainless thermostat only required a small buzz with the die grinder to fit snugly in the top neck which we fitted to our refurbished top tank. The muffler needed a few new pieces to be made and welded in.
We put fuel and water in the old dear, engaged the choke, turned on the ignition - 2 turns on the crank - wellah!! Away she went like a Swiss watch. We drove it around for half an hour. The water pump sprung a leak. We removed and cleaned the blocked grease nipple, gave it a few pumps - not a single oil or water leak any more. The radiator cap has lost it's grip but for what the tractor will be doing it's not an issue. Next is a good dose of coolant/inhibitor, an oil change, a small wiring tidy up and our 90yo dad has got his favorite farm mobility scooter ready for when we get warmer weather.
In total 3 days work for 2 happy campers.
Thanks again for your support lads. Gave us the confidence to tackle the task.