Hi Jing

Yes, I had a quick look on the web and it seems it is an 84V (or 82V) lamp on the 120V transformer.

It MAY be as simple as just changing the bulb for a 24V one, but it is going to depend what the rest of the wiring is. 250W into 24V is >10A which is a lot more than 250W into 84V which is only 3A. 10A over wiring designed for 3A could get a little warm! It isn't a complex system - just a regulated transformer and a bulb, so provided that all parts match it should be easy to re-build to the new voltage. However, if you are unsure I would take the problem to an electrical engineer - perhaps an auto-electrician, or perhaps electrical engineering at the local college - anybody who knows how to safely build electrical systems.

However I would probably consider plan A and get a 240v-120v converter transformer - easily available- big/robust ones used on building sites. As Richard says, get a 120V analyser and plug your 120V enlarger straight into it. May well be the neatest/easiest solution rather than messing around with the potential issues of converting a 120V enlarger (if you are not quite sure what you are doing).

EDIT just seen your remark re the fan - this could also be designed for particular voltage. Suggest external 240v - 120V transformer with a 120V analyser is definitely the simplest route