New valves are cheap compared to the rest of the engine - once you have the valves pulled for inspection I'd be tempted to replace any that hit pistons.
If the valves are suspected units due to contact.....the lifters must be considered as well. This is the first time I have ever seen true "first-over" pistons. I would go ahead and compare piston protrusion among the four. But, I would NOT pull those pistons unless you HAVE to because finding rings for those will be difficult......unless Hastings or some other off-beat. This is why you do not have much of a ridge.
pistons would depend on personal preference, they dont sound too bad, just make sure the rods aren't bent. I would only replaced them based on whether or not the engine needs overbored, which is decided by cylinder bore wear and compression numbers.First over piston would be the first size over stock piston, so a 77mm piston because 0.50mm over size is the "first over" size piston.