Where 'ya see a busted ring? If you're talking about #5 that looks like lighting, but if there really is an empty groove that would be quite the feat - that's not a missing piece, its an ENTIRE RING missing. I doubt it as that sort of event usually leaves LOTS of evidence all over the piston skirt and I don't see any damage on the visible parts of it.

As for the airboxes, yes, there's soot mixed with oil in there. But its not obstructing the ports; only one picture (the top one) shows material obstruction in the liner opening and its not enough for me to go tampering with it as it is not threatening to get into the cylinder or obstruct the ports. If the rings are not damaged and the engine runs ok I wouldn't change the kits and I DEFINITELY wouldn't try to clean the airboxes, which is almost GUARANTEED to get the gunk into the cylinders and do damage.

Make sure the airbox drains are open and check the rings. As I noted the easiest way to do that is to bar the engine so the rings are visible in the port and press on them with a wooden dowel. They should have a "springy" feel to them. If you have one or more that have NO spring to them that ring is either missing a chunk somewhere in its circumference, its stuck (if its slightly proud to the piston) or its broken (if flush or below) and that cylinder kit is toast. In that case there IS damage to the bore whether you can see it without a boroscope or not and the engine must be torn down. The oil control rings cannot be inspected as they never come to the level of the port, but a damaged oil control ring usually shows up as grossly excessive oil consumption and/or lots of oil spitting out through the airbox drains.

If you find one or more cylinders with one or more damaged rings then I concur that the kits need to be changed and due to contamination you probably need to do them all, rather than one as that will allow an airbox cleaning to be done as well - never mind that the labor to tear the engine down sufficiently to get one out makes doing them all not THAT much more work or expense. However, absent ring damage I wouldn't do it as the airbox, while dirty, isn't contaminated enough for me to tear the engine apart and replace the kits on that alone.

This assumes the engine starts and runs ok. If I had a boroscope handy I'd look at cylinder crosshatch condition but absent one if it starts and runs ok and the rings check out I wouldn't tear the engine down from what I see in those pictures.

That's my view on it; YMMV.