It seems to me, answering the unknowns would be helpful for starters. Running a bore scope in the cylinders may be worthwhile (looking at accumulated carbon & cylinder walls). Before you laugh to quickly at expense, you can get one that works with a smart phone for under $100 at Home Depot (Ryobi Phone Works Inspection Scope).
Not sure what you mean by the last statement quoted. Think there's more power to be had by increasing compression. By all means, update the manifold to an Air Gap.
IMO, there's no air flow to be gained by going to a late model or magnum head. Suggest a look at KB243 pistons for improved quench.