With Gard to replacing parts on a computer.
If you are sure the Mobo is the problem
The best bet is simply to purchase a Motherboard/Processor/memory bundle for some one like
However you might seek advice about connecting up
the hard drive as the operating system may not work because it does not recognize the new part.
If you are unfamiliar with all this
Then best buy a new computer, then you can remove the old harddrive and use it as an external back up and storage device