About the only way you can stroke the A15 is by offset grinding the crank down to the smallest undersize, then machine the surpluss material that protrudes above the deck from the rim aroind the bowl of the piston. It's a fairly big job for a very very small gain, & i'm talking about 1mm increase in stroke here. This amounts to about 1.2% increase in the stroke length,... at best.


The maximum increase in stroke, using factory parts would be defined by either the smallest undersize bearing available, or the strength of the material above the top ring, because there isn't a lot of it.
These engines will take a bit of a bore though & you will find that you can get a much more saticfactory result by following this route. Much more cost effective too.

About the only stroke job that is worthwhile in the A series of engines is the A14 crank in the 1200 /A12 block. This gives a 10% increase in stroke with no machining, but for most people, it would be much better to simply install the later & larger engine & save all the grief, as a successfull conversion involves a big bore job & some hard to get rods.