My best advice for this would be to blow up a photo of the GGB to a desired size and cut out maybe a third or a quarter of the image then spray glue it to a canvas. Use the cut out piece as a guide to draw the missing part onto the canvas then try to paint it back on using either oil or acrylic. To make it look even cooler, it might be a nice idea to use just shades of one colour on the painted parts to add a unique touch. I wouldn't paint over the top of a photo, I cant see that ending too well. Sounds cool, good luck with it!
