You can probably add the stuff for the head directly in the head in the layout.master file of your skin. I assume its just script files and maybe css. You will need to use urls for the script and css that are fully relative to the root of the site like /yourscripts/yourscript.js
ie that would work if the script is in a folder beneath the root named "yourscripts", the first / represents the root of the site.
If the map is supposed to be on every page you could put that div also in layout.master somewhere.
If you want it just on a specific page then you could add an instance of the Html Content feature to a page and in source view of the editor you could paste in the div.
Rather than set the onload on the body element you could wire it up from jQuery like this
This should be equivalent to body onload, it will fire the script function after the page is loaded.
The script should go wherever you put the div, ie if in the Html you can paste it also there or in layout.master if that is where you put the div.
Hope that helps,