Hi Rey,
I'm in fact rebuilding an early 1967 NK 1800, with the longer stroke and smaller bore (85mm 3rd oversize) I got lucky and found some NOS mahle pistons for my application, an ebay seller called Linea Veloce has few in store and at good prices, I got mine for 290$.
http://stores.ebay.com/lineaveloce
I'm changing the 1,8i head with a 118 korman stage 2 head, also found on ebay that has the matching combustion chambers for my piano pistons.
Thanks
Omar