Those are at a big disadvantage -- too small -- for the 1.6 liter class rules. And they are too big to be turbo'd under the class rules

For naturally aspirated I would consider the 1597cc Nissan QG16DE, comes in the Pulsar N16, 4V head, twin cam with variable cam timing. Fit flattop pistons, order custom cams from a cam grinder and go to town