Did you check out Wiseco???
They have LS650 pistons. If you know the bore, maybe you can contact them and ask if they have equivalent.
http://www.wiseco.com/here's their LS650 numbers.... but I though STD was 94mm
LS650 Part Number Price BoreSize/
OverSize Attributes
4597M09500 $182.18 95.00mm /
STD Displacement: 641cc
Stroke: 90.40mm
4597M09600 $182.18 96.00mm /
1.00mm Displacement: 654cc
Stroke: 90.40mm
4597M09700 $200.41 97.00mm /
2.00mm Displacement: 668cc
Stroke: 90.40mm