The reason why a new motherboard would not work with a "custom" power supply is because they normally put out a low amount of watts, because of that the are comprimised on size most the time.
My old HP one needed replacing because it was only 115w, and because a full size one would not fit in the tiny case i went out and brought a proper cade and 250w power supply, and every one told me it would not work or fit, HA it did! and ran fine for as long as i has it.
The standard running gear dosn't need 300w psu so the crappy one do just fine for the job, untill one starts to expand.....