There are lots of old taxidermy mounts out there - garage sales, antique stores, ebay are all good sources for older pieces. You'll probably want a high quality mount since you'll be removing the bulk of the internal structure, so better quality (ie not falling apart) probably means stronger.
Step 2: Purchase computer components
Purchase your computer components. You'll want to do this before you start so that you have a general idea of how much room you'll need to create inside the beaver, and what sort of slots you'll need to cut for specific components. For a basic machine, you'll need a motherboard, a hard drive, some ram, cd or dvd drive, a fan and a power supply.
Step 3: Purchase a motherboard
There are a variety of boards you can choose from. I went with a MiniITX board because of size - it's 17cm x 17cm versus the much larger standard motherboards. If you have a HUGE beaver, it would be possible to install a standard board, but you really need to make sure that it will fit. Another plus with the MiniITX board is that it included an external power supply, which means a good bulk of the heat will not be inside the beaver. Good news, trust me.