Your block may not be drilled for hydraulic lifters. Your only choice may be a solid cam.
When was the last time the valves were adjusted?
A loose valve may be what caused this.
Did you use an additive or
oil with ZDP? The lack of ZDP in modern
oil has been rumored to wipe cam lobes on flat tappet lifters. There is something to this, but perhaps not all claims are totally true.