Presumably, if the enlarger originally ran off a US 120VAC supply, it has a transformer in it, or other electronics (voltage divider, regulator, etc) to provide the lamp with the correct voltage. If you were to replace the lamp/supply with a 12V system, you would need to bypass all that: you would connect the 12v transformer between your timer and the enlarger.

An example of a Halogen transformer could be the one Raoul linked to, or a domestic switch mode device like you find in the ceilings of modern homes - as long as it's rated at the same current (domestic transformers often supply 11.5V instead of 12, and so the lamp lasts longer, runs a bit yellower!)

Marc!