This week I have been mainly rebuilding a Polo 1.4i engine (and been to work + spent a day and night in hospital with the young un on top

The hydraulic tappets were very worn (0.5mm dished)
This led to one having a catastophic failure (shrapnel everywhere etc..)
Tappet died about 6 miles from home on a dual carriageway
In the process of getting home the broken tapet chewed the lobe off the cam, and chewed the end of the valve stem, and scraped it's bore.
The tapet also held the inlet valve partly open.
The head is a single piece so the entire head needed replacing due to the damaged tappet bore. The upside was that a new head negated any issue with the damaged valve.
So with a new* cam, tappets and head, + head gasket + fresh oil etc.. we figured it would be fixed...
No!
During rebuilding we spotted that the end of an injector had been melted (which was how we guestimated the inlet valve had been left open)
So when it ran rough we replaced the injector (local scrap yard).
Now it ran smoothly but it was not idling and would surge from 1500 to 2500 rpm.
The answer? - anyone thinking air leak is probably an experienced mechanic!
In desperation I took off the plastic inlet manifold to investigate the seals etc looking for an air leak.
And found that one port was burnt up and had a few small holes in it

By this time, 2 evenings and 2 staturday afternoons had gone, and it was too late to head to the scrapyard to scavenge more parts so another week gone, £110 spent and no car on the road to show for it.
It it had been an A-series engine, the worn tappets would have made it run a little bit noisier. THATS IT AND NOTHING ELSE!
Thin skinned hydraulic tappets = very poor
Single piece head = understandable but less maintainable.
melted injector = annoying
failed plastic manifold = useless!
Long live the trusty Morris Minor!
*new - actually they were 2nd hand but low mileage warranted parts from a VW breaker.