Which plugs are best for this engine?

The biggest differences between the K6A end F6A is K6A has an all new Aluminum block and head, whereas the F6A is cast iron, this helps with the heat soak issues that altos struggled with. And as you mentioned, timing chain over timing belt. This one is nice condition, it doesn’t have VVT tho. Which can be a plus or minus however you decide to look at it. As an econobox the VVT is great, turn on ECO mode and it advances the timing reducing boost and horsepower by delaying it until further in the power which gives better fuel economy. Non VVT’s are better for performance driving because Suzuki Sports never made an ECU upgrade for the VVT engines. I have a VVT engine in my HA22S with ECO mode and I wish I didn’t so I could plop an ECU in there and make 120hp.