Was it ever backfiring, popping back on the over-run, misfiring, hesitating upon asking for more with right pedal, sneezing through the carb, refusing to rev ?
Can we see the plug please?
Will reserve judgement based on answers above, but tend to agree, doesn't look or sound like weakness
So who was driving it when this occurred, what ist 900 mile history consist of, when was something first noticed
e.g. was anything heard, seen, felt and if so what, when & who/
More clues to what when wrong and when seem to be needed. Partic, whether the engine ever had full compressions and ran properly on all 4, was it starting to bed down from rebuild, was it pushed too hard too early (heat).. what zylinder is kerfooked?
Need a better closeup, but does look as if detonation might be involved, the way the crown is eaten away, though don't have much VW air-cooled experience. Strange though just that one cylinder, can we see piccies of the other piston tops?