I bought an A15 from All States Importers WA for AUD$575, and bolted her straight in. The A15 is identical on the outside to the A12, as far as bolting up goes. The A15 just has a bigger bore and longer stroke.
Only dramas were exhaust (Jap engines have a 3 bolt flange, Aus have a 2 bolt) and I had to muck around with the clutch cable, it needed to be longer.
Ok, *YES!* there is a difference between an A12 head and an A15 head. Pictured is an A12 head. Note the circular ports. An A15 head has oval ports, and cooling jackets - check out the photo of the carbies.
The non-water jacket A12 head.
So the bottom line here is if you are going from A12 to A14 or A15, you'll need the exhaust and intake... Alternator, dizzy and starter are the same, note however that the dizzy curve may be different.
I have no comment on the extra power obtained
from a 25% increase in engine capacity ;)