There are a bunch of variables, and the wide range of responses shows that.
Here is what I will add.
1) If you need to put close to $1000 to fix the guides, valve job, resurface, replace parts, etc. Then just buy aluminum heads. You will tolerate more CR & make more power.
2) Your combo has several things that should help tolerate CR, but still your experience shows your current CR is too much. The timing curve change can help, but may not be the only thing needed. If you could reliably control the timing curve to come in much later (3000 rpm?) it stands a better chance.
3) That 68cc chamber is a problem. 10.0 CR should work with 93 octane, and no need to push it over 10.5 with iron heads. IMO