Andreas, doesn't look to bad from need but has you've stripped it down you may has well do it, not sure if it's still the same with modern oils but I was always told to leave a ring of carbon closest to the piston edge to aid sealing . Leak must have been like like that for some time and then left for a while ? The gasket does show more heat around the pushrod tunnel if my eyes don't deceive me so if solid anneal,, and lastly after first run out whizz the rocker box off and retighten the bolts to a similar torque bill mentioned ,can't remember exactly what it should be but within a few mins someone will post. You'll sort it and you'll already have had your first experience of BSA's . It's abit of a pain getting the pushrods back in there cups ,you can buy a special tool for about £3 but others use various tools fingers etc to engage them, don't be put off you'll soon master it ,regards BobH.