I just did a head gasket replacement along with bottom oil seal on a briggs 20hp intek twin. after cleaning head and motor surface super clean i reinstalled the heads with new gaskets, cleaned head bolts and torqued to factory specs and after 15-20 minutes of mowing the right side facing mower started leaking, oil filter side.
mike, sounds like Il Engine has it. Head warpage is usually checked with a machinist level ground to .0005" flatness which you may not have. Cross and diagonal checks are made. A feeler gauge is used as a go-no go . Allowance would be very small, like .001"? I don't think surfacing is done on these little heads. Cylinder warpage is trickier I would think as it could be twist and /or height. Perhaps Il Eng may chime in, but seems like a small engine pro might have to make some measurement for you before you throw more money into parts.
Get a .003 feeler gauge, lay the head on a glass window, and if the feeler gauge will fit under the head anywhere around the perimeter, toss the head and get a new one. The warped block at the cylinder head mounting surface is tricky to check. I have taken a known straight piece of steel and wrapped fine sandpaper around it and then polished the mouting surface, If there is any spots that the sandpaper doesn't touch you know that it is not flat and will need to be worked on.
Had a single cylinder a few years ago where I had to start with 60 grit sandpaper to level the block where the head attaches, and then finish with 400 grit to smooth the surface.
