Yes it can be repaired to a point. There is a tool called a reseater which grinds the valve seat to make it smooth and grind out fine cracks. The seat can only be reseated a number of times before its too flat in which case the tap would then need to be replaced.
The cost of a valve tap repair will vary based on various factors. If you do it yourself, it may cost about $40 but if you take to a profession mechanic, you can pay up to $60.
Through the spark plug hole, soak the valve with penetrating oil. After sitting a while, tap lightly on the valve stem with a hammer. If after several tries this does not work then the head will need to be removed to see why the valve is stuck.
tap valve
3-3 in economy (easyjet,tap Portugal etc)
Valve
it is a valve, which is not a simple machine.
plug, tap, gate, pipe, lid
plug, tap, gate, pipe, lid
A good valve should not leak anything.
Replace it.
That depends on the design of the valve.
Spray wd40 or equal and tap valve "gently" with a hammer while "gently" appling turn pressure on valve.