mIRC has a line limit of ~4000 characters and the responses are often too long to fit. The easiest way to get the full information is just to open the relevant URL in your web browser, then you can examine the JSON structure.
Look at the user commands I've added to the script, you can see how I access specific data:
var %matches = $CoD.Matches($1,$2,$3,$4)
var %match = $json(%matches,data,matches,0)
var %kills = $json(%match,playerStats,kills).value
var %kills = $json(%matches,data,matches,0,playerStats,kills).value
I've made some more changes to CallOfDuty.mrc and changed the $CoD.Call alias so that it opens the URL when debugging is enabled.