Mazda have a lot of pistons in the 78 to 80mm range. If you want to use cast pistons then the ACL catalogue can be downloaded from their website. Check out the B6, E3, E5 and UC, just off the top of my head. You will need to increase the small end of yoru rods from 19mm to 20mm, ask your local performance shop. You will need to know what compression you want, and before that you will need to know your cam specs. Then you will need to work out what valve resesses you need in the top of your pistons to ensure adequate clearance. I use 0.060" of vertical clearance and 0.040" of circumferential clearance.

I've also heard of galant pistons with 19mm pin, but can't remember wether that was an A14 or A15.

Ernie is selling 80mm forgies - check out the classifieds.