Changing Mouse cursor, can anyone help me fix this?

Trying to make it when you hit Z once it changes and Z again to back to the basic: what i currently have is listed below

wait()
local Tool = script.Parent
enabled = true
zoomed = false



function on()
if Mouse then
Mouse.Icon = "http://www.roblox.com/asset/?id=183767324"
end

function off()
if Mouse then
end
Mouse.Icon = "http://www.roblox.com/asset/?id=183710147"
end


function onKeyDown(key)
    if (key~=nil) then
        key = key:lower()
        if (key=="z") and not zoomed then
                on()
        end

            if (key=="z") and zoomed then
                off()
        end
    end
end