Have a good look all round the pan rail surface, the face, inside and outside. You're looking for cracks.
The valves should be easier than the old type because you can use a conventional valve spring compressor to get the collets out. You can then tap the guides down a bit then pull the horseshoe clips out and then pull the guides out. Keep them all as matched assemblies.
If you check mart's garage on youtube I did a complete teardown and rebuild on a despatately crusty flatty. It was an earlier type.
You might find the videos entertaining and from what you have said, yours ought to be a much simpler proposition.
Mart's garage:
https://www.youtube.com/channel/UCzc...Vro40j-vjjZShg
Mart.