I would recommend getting some references off the web or from the Halo art book and just starting over. The chassis is not the right shape and the proportions are off. The circle you used for the wheel is too blocky and bumpy. The barrel of the gun is too small. The windshield is the wrong shape, size, positiion and color. The roll cage is not even there. The "tusks" (for which the warthog is thusly named) are absent as well.
