The way I would do it (in Photoshop, I know nothing about Gimp) would be to put the hood layer above the engine layer. Then add a layer mask (white/clear) to the hood layer. Then I would use the brush tool on the layer mask and 'paint the mask on' with a soft edged brush. I would paint the areas that I wanted to mask on the hood layer...the areas that I want the engine to show through...with black. If I wanted to cover an area back up, I'd switch to white and keep painting.
Here are a couple I made with my Blazer a few years ago