What kinda temperatures does an engine produce?
What material are billet head domes made from?
I don't know the billet question and the temperature question is very broad...
In a jetski engine the exhaust temperature is well over 1000°F but the outside of the pipe, case, head, etc. should all be cool enough to touch without burning yourself (aka less than 200°F) thanks to water cooling jackets.
I think the real question is why do you want to know these things? There are some smart guys on this forum, let us know what you're thinking and someone will most likely chime in.