Carbon buildup on the dome of a piston is completely normal, it's a byproduct of the combustion process. It can always be cleaned off with a wire brush on a grinder, but just be careful to not damage the piston in any way - particularly the ring lands, or grooves. Are they the proper size for your engine in its current state? Stock bore? You should post some pictures of the sides of the pistons as well, and you should check the clearance of each one in each cylinder to see which piston fits which cylinder best

 

Hey Tom ...

Thanks for the advice ... glad to know it's workable ... here are a couple of side views ... there's a tiny bit of scoring on one side of one of the pistons, but it doesn't look too bad to me ... Bill

 

They look serviceable... check the clearance of each in each cylinder with flat feeler gauges (or have someone measure them and the cylinders), and when you get new rings for them (and you should) you should check the clearance of each ring groove to be sure they're not sloppy, as well as the end gap of the rings in each cylinder. Use the top of the piston to push each ring into the bore squarely, down into the area of piston travel, before you measure the end gaps