So, Following ways, you can cool the exhaust valve,
The water flowing from these jets or nozzles is in contact with the areas adjacent to the exhaust valve and exhaust port, which are thus cooled.
02) An improved cooling can also be achieved if cooling jackets are provided more elaborately near the exhaust valve and exhaust port.
03) By filling a part of the stem of this valve with sodium, you can achieve better cooling.
The solid melts at a high temperature (Near about 750-degree centigrade).
It is thrown up and down due to the motion of the exhaust valve.
The heat from the hot region is thus conveyed to the comparatively cold region.
Sodium is a good conductor of heat and has a high boiling point. (750-degree centigrade)
Hence it is more important to keep all the parts within workable limits for the better efficiency of an engine.
Besides this information, you are suggested to read something more from below engineering books
To get the more details about the topic, I further recommended reading