It sounds like the builder was relatively clueless about engine work, as many custom shops are. While they build beautiful cars they often just bolt things together when it comes to engines and didn’t adjust the rockers correctly – which would explain clattering.
I would at a minimum pull the heads and check some things. Is the pushrod from that cylinder bent? That would tell a lot. A collapsed lifter could be the result of a bad lifter, but more likely caused by any number of interference issues. Check retainer to guide clearance, valve spring stack height, look for signs of piston/valve collision, and check valve guides for too little clearance – a problem some aftermarket heads are notorious for.
I’ve never seen a valve tip worn like that before without there being actual breakage of a rocker or roller tip. It could just be poor quality control on the valve finish, and lash caps would preclude replacing the valve. Also, what brand are the roller rockers? The cheap Chinese ones look great until you start to run them…. Then they fall apart and destroy your engine.