How much can I modify the design and/or look of a Grain Homegrown Kit?

Plenty. The main restriction is that the shape of the board is defined by the computer-cut internal frames. You have some design flexibility in shaping the rails, extending the length (with a longer tailblock), making a pintail instead of a squashtail, making the nose finer or blunter, and some other areas. Of course, much of your board's identity will be determined by how you decide to "plank" it. You can inlay other woods or designs, stain planks, paint accent stripes, and even add colorful fabrics beneath the fiberglass.