Sounds like someone didn't drain the coolant first.
Just so you know next time... The head bolts should be detorqued in the reverse order of torquing. Failing to do so
may induce damage to the head or the threads in the block.
Engines can build up crud on the piston tops for a variety of reasons - blow-by, rich running, inefficient combustion all spring to mind. You need to troubleshoot, and from the nature of your other posts, I urge a slow, measured approach from here in. You're going to invest time and a few new parts; why not get it right. Take the carbie episode as a pointer that you may be diving in a bit hastily.
Inspect the block. Look for tracking of combustion gases or water under the gasket contact areas. Lay a straightedge along, across, and on the diagonals of the block. If you see daylight under it, measure with feeler gauges and record the number. Feel the top of the cylinder bores for a "ridge" where the wear stops; note about how thick (this is by guessing/visually comparing to feeler gauges). Pay special attention to #1 bore, also the rear two.
Scrape the piston tops clean with a wood scraper (maple sharpened to a chisel point is good); you might want to use a little brush-on type paint stripper if it's really thick. Always clean the piston at the TOP of its travel. That way, as you lower it down after, you can mop up all the little bits which were stuck hard in the edge. You can clean them in pairs this way, as they come down look for scoring, and whether you can see any honing marks still. When cleaning the piston tops, plug all the other holes with rags or paper towel. No point contaminating your oil supply or a just-cleaned bore. Preserve the bores with a liberal swab of clean engine oil and a new white cotton rag popped lightly in the top.
Time to focus on the head. IIRC, yours has some broken bits? Maybe pick up another unit - CC'ing if you do, and check your theoretical compression level.
With the head, it is especially important to keep everything in the correct order (did you do this with the pushrods?). Again, I like methylene chloride paint stripper for degreasing and cleaning up. It is highly toxic, so be safe. Caustic soda solution works well, but for whatever reason is likely to film-rust immediately, where the stripper doesn't. Clean the head initially, then dismantle and clean again.
If possible, I suggest you next pop all the freeze plugs, including the log ends, and get the head insides glassbeaded. This will remove almost all the rust scale from the water passages, and the carbon from ports. The glass beads will not affect the valve seats where garnet or sand abrasive may do significant damage. Even so, ask the blast guy to go light on the valve seat areas. Most blasters offer a phosphate coating spray that protects lightly against corrosion for a few weeks - consider this, too. clean the head thoroughly of all abrasive and let it sit a day or two. Any cracking will show up as a stain line, where oil weeps back to the surface. Hopefully, there isn't any!
Next, check your valve stem clearances and seats. Worn guides can suck oil, worn seats lose compression. Compare to working limits. Wear in these areas will determine if machine time is needed. The valve stem oil seals are cheap and replaced as a matter of course. Hopefully, you don't need anything more substantial than them. (Unless you are wishing to mill the head for either flatness or a compression raise.) Reassemble with engine oil as a lube, paint the head and reinstall. Note well the rest periods required when torquing down. I place assembly lube on both ends of the pushrods, and the valve stem tips, always.
Start it up with new, correctly gapped plugs (don't trust them out the box), and a reliable carb (borrow one?). Then report back...
Sure, it's some work but you'll learn as you go, plus save on what the machine shop will charge.
Cheers, Adam.
