Have a read through this ( I dont think Amal will mind 
)
It is an excerpt from Amals RFID Toys… book ( That I own, by the way) I think it is out of print but you can still pick up 2nd hand copies from ebay / amazon
Where this refers to KL042, just replace that with xAC V2
key fob.pdf (342.4 KB)
We would need to look at voltages, but this is pretty much what you would be doing.