On a 2.5l the tone wheel, looks like a gear, is press fitted onto the back of the balancer, so shouldn't come off but have seen that before.
You should replace balancer, either new or from a wrecking yard
Reason why is that the CKP(crank position) sensor is the Big Kahuna of sensors, your engine will run with any other sensor unhooked or failed, except the CKP sensor
And its timing sets spark and fuel injection timing.
Cam position sensor(CPS) just fines tunes CKP signal.
Even if you got a new balancer and then used it to line up the old one, then returned the new one for a refund, you still have a very good chance it will loosen up again, and strand you.
It also needs to be exactly in the right position, within 1deg, and as you have found out "close doesn't count" with CKP sensor timing
Found this image of balancer: https://www.carid.com/ic/dorman/products/594-418_6.jpg
Looks like the "missing tooth" is about 10deg off of woodruff keyway
Good images here as well: http://www.ebay.com/itm/Harmonic-Bal...-/301830520891