Here in Japan they modify the H72 early round port head into many variations
one is a high port and the other a big round port

either way you can use your stock A12 head and get 100kws atw with
a simple well tuned turbo drawthrough or supercharged with 1.75 su

The stock a12 heads have more flow than mini heads yet minis with
cast iron head make 100kws atw with good boost from turbo or supercharger

I would recommend you just refreshen your A12 and go for a toyota
sc12 supercharer and 1.75 stromberg type SU good headers, igntion
and good tuning. John Adelaide makes some nice pulleys and adaptors
but would steer away from amr500 if you want good power as the sc12
matches capacity of the a12 and needs less rotation for good hp.