It could be anything, tune stands out to me, as well as various cooling parts, but since nobody has touched on it yet...

Has anybody thrown a block in a hot tank and actually CHECKED for silt etc at the bottom of the block?
I have and have found some blocks packed with hardened debris. I used all sorts of tools to dig it out through the frost plug holes, I was amazed at what was left in some...

If all else fails it *might* be worth checking as a very last resort..? I have a feeling timing and carbs might be the first area to look at though...