The command line utility
bc is capable of arbitrary precision mathematics, and you can call a command line utility from AppleScript and get a value from it.
set myVariable to do shell script "echo \"89723443578920345892 / 2\" | bc"
Gives this result:
To pass command line arguments as AppleScript variables:
set myNumber1 to "89723443578920345892"
set myNumber2 to "2"
set myVariable to do shell script "echo " & myNumber1 & " / " & myNumber2 & "| bc"
But this gets us back to the original problem, AppleScript represents large numbers as a float, so I had to pass the value as a string. So depending on where you get the very large number from, this could be problematic or a non-solution.