Marc, your beautiful Duetto is progressing nicely. You have a great shop/garage and very eclectic taste in automobiles. I am sorry about the head work and was concerned that replacing the valve seats and guides would be an issue.
My Duetto engine and transmission are out of the car and the engine parts are scattered all over the garage as I am trying to decide what to replace and where to get it. Several suppliers offer 'OEM' pistons and liners but I thank Borgo and Mahle supplied them originally. I see yours are Niiral, which I believe Classic Alfa supplies. I doubt that they were an OEM supplier, perhaps the compression ration 9.0:1 is 'OEM'. Can anyone here vouch for Niiral pistons and liners; is one type better than another? How about rings? It seems like you always need to use rings other than those that come with the pistons...
I plan use Viton o-ring seals everywhere possible (with roll pins already in the block). The alternatives rubber seals all seem to dry out, shrink and leak! Viton seals are available as an upgrade to the engine 'kit' or they can all be purchased separately.
Engine OH 1600 9.5:1 STREET COMPLETE OEM Pistons - Spruell Motorsport, Inc - Alfa Romeo Performance Your Store for Sports Car Performance and Racing Parts
VITON Head Gasket O-Ring Seal Set (6). Add $7.00.
VITON Ultimate No Leak Engine O-Ring Seal Kit. Add $19.00.
6 head gasket double seal o-rings
1 front cover double seal oil passage o-ring
1 oil pump o-ring seal
1 distributor o-ring seal
Liner seals (?)
Just for the record, Paul Spruell is my kids 'great' uncle. I have no commercial interest there and likely receive no discount on parts I buy but strongly recommend Viton wherever you get it!
Oh and while we are at it, we should both replace our nasty ground straps as shown in post 10, third photo down. Dirt and corrosion diminish their ability to provide a good ground (as does paint and undercoating, if your strap was removed while the body was prepared and painted).