Been there fixed that a few times. As mentioned above the oil rings jam up and no longer scrap like they should. If u dare get it to op temp it will reduce the smoke but theyres no additive that will fix it.
Can be done in chassis in a m100, head off, sump off. New rings are required most likely standard but double check bore. Pistons are slavageable generally. Comp rings usually come off no worries . Oil rings might need digging out be careful not to damage piston. There are 4 holes that need cleaning out. A 2mm drill bit sorts them out. Carefully scrap the carbon out of the grooves i snapped a top ring in half to get most of it out. Then a flathead screwdriver just a tad smaller than the groove to get a bit more. A gentle wire wheel to clean up the rest and ur done. Use of carby cleaner is recommended too. Lightly hone the bores to remove the glaze. Dont hit the crank as it will damage the stone. I allways put a new set of big end shells in too.
If your your doing valve stem seals make sure u dont mix up which valve came from where. Setting up valve clearances can be a pain but still check them.
Ive never come across a corroded head yet.
The upper water pump o ring isnt in a vrs.
The job isnt that hard, just time consuming if u havent done one before.
Hope that’s a bit of help to someone