The general case is that you can't mend a transformer. The usual failure mode is a winding (wire) melting deep inside.
Any old transformer will do - usually... All you need to do is to pay attention to Input Volts (115 / 230 depending on where you live) Output volts (usually 12V for lights - your bulbs will tell you) And WATTAGE i.e the total load in Watts of all the lamps being supplied by the transformer plus a little bit more to be on the safe side.
If you match the load exactly the transformer will probably work OK but not for long - always better to have a few Watts spare to prevent warm running and premature failure.
Peccavi, April 2010
