The PSU just needs to supply +5v and +12v. Not too much of a tall order to find something suitable and make holes in the bottom moulding to fix it there.

ISTR the 12v is only used for the HDD/CD power and is regulated down for the audio. 4A is probably safe. 5v is used to produce the 3.3v rail and 2.0v CPU core, plus run the 5v side of the HDD/CD but again ~4A is probably ok too.

Maybe one of the mini-ITX PSUs would be suitable?

Hugo