Hi Guys
Here a used set of Omega pistons 80mm diameter in good conditions.
http://www.ebay.co.uk/itm/Lotus-elise-rover-k-1-6-1-8-Omega-forged-pistons-/110876318090?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item19d0be598a
All the best
Vicente