Well the A15 actually IS an A14 with 5mm more stroke. The only real difference is the longer stroke crank, the shorter deck height pistons [same rods] & an oil pan that has bulges on one side to clear the rods.
To stroke the A14 will involve engine removal & the instalation of the replacement crank, pistons & sump from an A15. While you're at it you would be seriously remiss if you didn't at least lap the valves, give the bore a very light hone & renew the rings & big end bearings & mains, just to ensure that the finished engine had full compression & oil pressure when it's fired up again.
Installing an A15, which is an original fitment in these anyway, would be simplicity itself, particularly if the replacement was sourced from another Sunny wagon. Either way, stroke or replace, you're going to need an A15 short block as a bare minimum, even if it's just a donor for the crank & few people break up a good running engine, so a donor short block may well have some worn parts in it.
I have been asked to post some pictures of the pocket rocket, so here is the Sunny in question. Enjoy.


OK, I left out another side shot but you get the idea.