[Stefan]

No , dont take the engine out.
Youll need to bribe someone to lend you the special, hen's teeth head spanner and copy the instructions from their manual. And a gasket set (from BCL )

Remove carbs, plug leads, fuel banjo pipe.
Remove carb bases.
Drain rad and remove water pipes
Undo head nuts.
Remove exhaust manifolds.
Lift the head off, and take it to a reputable motor engineering workshop to check for distortion, leaks,pinholes and damage. They will then need to planish it removing the absolute minimum of metal; it will probably be scorched between the 2 cylinders where the gasket has gone.
Consider fitting lead free valve seats and having it resin sealed. Look at the oil cross tubes for leaks, they are buggers to keep leakproof.

New heads can be bought from INskip Racing, but they cost THOUSANDS so cherish the one you've got.
Check that coolant is draining freely from the bottom block tap. If not the block is scaled up and you should take advice about having the block's water jacket chemically descaled and derusted, otherwise it could well overheat again.
Check head nuts and studs for damage, corrosion, and replace if necessary with new.
Reassemble , tightening the nuts in correct order as per manual, using the special spanner on the ones you cannot reach with your socket set. Tighten these ones (as you cant get to them with your torque wrench) " just right".
Reassemble carbs etc, insert coolant, drive it - some say 50 miles, others 500,, then take the carbs off again and retighten the head nuts.
Have I forgotten anything?
My record is 90 minutes to get the head off, and 3 weeks to get it back on....
Good luck! Take great care, and it will be repaid with good results.