I know its not perfect but then again, Valve can care less lol

+More Pull Back the More Damage
+Crits in the head if pulled back all the way
+10% Ammo
-No Random Crits
-15% Reload Time (You are reloading a crossbow)
-5% Fireing Speed

With Crits head shots can do 700 damage instead of 400, Body shots would do 100-250 instead of 32-125 (The amount of damage can change based on the pull back on the crossbow)
Pull Back:
Weak 32-55
Decent 56-85
Strong 86-125

Now this is NOT perfect but its a start.