Can I use an over-rated transformer for a low-voltage lighting circuit?
You can use either. 

54 watts is the load from the bulbs. The transformer must be larger than the load. 

In theory you could use any transformer 54 watts or greater. In practice though inefficiencies such as splices and voltage drop mean that exact 1:1 ratio doesn’t work anymore, so upsize 20% to take into account inefficiency. Meaning in this case use any transformer greater than 67 watts.