compession with after market pistons

If you don't have the piston part # and specs you could cc the bore with the piston at TDC if they're flat tops but pop ups would be far more difficult to figure out. You would have to make some kind of plexiglass tube with a known volume, grease the bottom for a seal and cc it again while placed over the bore and piston. Then put your numbers into a calculator like this one: http://kb-silvolite.com/calc.php?action=comp