The only way I can think of an a15 fitting an A12 block is by using small crankpin style rods like hayabusa or other bike rods when you offset grind the crank and keep the 82mm stroke or perhaps using hayabusa rods with 37mm ends youll be able to run 84mm stroke. The extra 5mm or more can be overcome with say rods with to suit crankpins of 40mm or lesser. This will allow the crankpins to clear the sidewalls and also I think Chris used an a15 sump?
However the cranks will need machining to stop the pistons hitting or you will have to use slipper style pistons from bikes.