Well folks here is some info for what you want to do. The A14 or A15 head will fit as other posters have said . But the Book says that the early A12A engine has torque @ 33to36 ft.pounds or
4.5 to5.o of torque .the thing is that the head requires a spacific amount of holding down hence the A14 or A15 heads need the amount of torque for the particular head or you will blow
gaskets ,it also causes proper gasket crush do you follow? so because the A12 head bolts torque where they do and the A14 head bolts are torqued @51 to 54 ft.pounds or7.0 to7.5Mkg you will
need A14 bolts to hold an A14 head and so on . the A12 bolts were not made to torque that tight and will stretch then fatigue and ultimatly fail if they don't strip while tightening them . It
all adds up to a warped head or blown gasket (NOT GOOD!!).
My book says that the A15 head bolt torque is the same as the A14.
Ok now the following is My opinion so take it as such! A14 ,A15 head on A12 will get you this : BIGGER valves Equaling more flow (GOOD!) also bigger ports and so on BUT!! they also have a
larger volumn combustion chamber so you will loose compression (NOT SO GOOD) you loose H.P. and torque. To catch back up and to really get the best benifit from the bigger head you should
make sure you feed it better and regain the lost compression. This requires a taller and longer cam,flat top or domed pistons or shave the head
Milling will get almost a 1 point gain if you cut 90 thousanths no more, but milling is not the best way to gain compression new pistons do it best.The other thing that needs tending is to be
sure to give it more carb or it will run lean and you will end up with an unhappy engine . If Money is a consideration and it usually is! Try a pocket port and polish on the A12 head ,Port
the intake and use an A14 carb that has been polished and you can thin the throtle butterflies to get more flow but remember that if you up-size the jets that the air bleeds need to flow more
or it will tend to run too rich and not reach it's full potential . well, good luck and post any tips you have for me I am always willing to learn more
Thanks
Steve