I use NX5 pistons in my race bike. They are shorter, so you need to skim the cylinder height down to suit. Its a lot of work. An easier option is the 90s RM125 motorcross piston which is very similar to the NSR and a lot chaeper than an NX5. See a thread on this in the main discussion section of the site.

On the A kit piston, the ring peg is at the rear of the piston rather than off set. As Kieran says, it runs over the rear transfer port itself. This is possible as the A kit piston uses a very thin ring (0.7mm) so is flexible enough not to snap like the more rigid 1mm standard ring.