Hmmmmmmmmm.  Is this it:http://www.rockywoods.com/Hardware-Zippers/Buckle-Hardware/2-flat-metal-slider-buckle-Black?

If so, then there's another question:  Is your original buckle sewn-in, or can you detach the original buckle simply by unthreading the webbing?  If the latter, then just remove the old ones, and install the new.  Paint 'em first, if you like.

Now, if the old buckle was sewn-in, they you will need to do some work.

First of all, tale a look at this field-expedient buckle.  It's a different type of buckle, but that doesn't matter right now.  http://www.rockywoods.com/Hardware-Zippers/Buckle-Hardware/1-1-2-Split-Bar-Side-Release-Buckle.  When installing, needle-nose pliers will be a big help.

See the part of the buckle with a diagonal split in it?  You'll need to re-create that split in your metal buckle in the part that's currently sewn-in.  I'd use a rotary file in a dremel tool, and smooth all the sharp edges afterwards, lest the sharp edges cut up the fabric on the rig.  I advise getting more buckles than you need, to cover yourself if you make any mistakes.