Imac G4 Power Supply Swap

I got a 15" imac with the 700mhz processor for 40 dollars on craigslist. The power supply is bad i have already determined that. I have power going to the power supply but none coming out of it. There is actually a black burnt spot on the bottom circuit board so i know this is the prooblem. The question i have is i can't find a 700mhz power supply for it. I found a 1ghz power supply on ebay though and was wondering if i could put that one in it instead. The boards are the same size as far as measurements so they will mount with no problem but is the logic board gonna be affected by a bigger power supply. Im guessing its only more power because it powers a bigger screen and processor....I read that the logic board will only use as much power as it needs...is this true????

Thanks for your help