a 20R head does not raise compression by any decent amount! the chamber is only a few CC smaller than the 22R open head.

i swapped one on my '82 22R bottom end, since the head was warped, and busted a valve guide(sick of fixing the thing).  i put an EB 268c cam in it, and engnbldr's oversized valves(end up just smaller than 22R valves but have correct rocker geometry).  also had the machine shop do a mild port job near the seats to get more benefit out of the valves.  then i put an offy C series intake on with a de-choked weber 32/36, and a thorley header.  with the correct jets, it really flies!