I am going to reuse the single with the motorcycle in it that I used to create my clean plate. Add the image into your composition and trim it to start at the same time as your actual clean plate
Turn it into a 3D layer and copy the position, orientation and scale from the clean plate layer onto this new layer. Do this one by one and ensure all transformation properties are exactly the same so the two layers are placed at the same position in 3D space.
It should now roughly look as if the motorcycle was back in its original spot.
We will duplicate this layer layer in a moment to create the charred remains of our motorcycle, so let's darken it a bit. For this, apply a hue & saturation effect, de-saturate the layer almost entirely and bring down the master lightness.
Trim down the motorcycle layer to start exactly at the moment the clean plate layer becomes visible.
We will now break up the Motorcycle layer that sits on top of our clean plate into chunks of the motorcycle that can then be flung around by the power of the explosion. Let's call the layer we just added 'chunk 1' and use a mask to cut out a big piece of the motorcycle. I will cut out the left half.
Next, we want to animate this chunk to be thrown around by the explosion. Before you start key-framing the position and rotation, grab the pen behind tool and make sure the anchor sits exactly into the middle of the fragment.
Then animate the position and rotation to make chunk 1 break off the motorcycle as it darkens. Ensure it does not accidentally slip behind the cover up layer (in 3D space) during the animation.
Here is the sequence of the first chunk of the motorcycle being pushed away towards the left side as it chars out.
Now duplicate the 'Chunk 1' layer and call it 'Chunk 2'. Delete the mask and create a new mask around a different part of the motorcycle. This time, I will use the right half excluding the wheel, which I will split off separately. Like before, move the anchor to the centre of the chunk and animate it to be pushed to the side by the power of the explosion.
Repeat the same process for the front wheel. Rather than have it drop off slightly, I am going to have the wheel spin wildly and be flung out of the scene on the right to make the explosion appear more energetic. And again, let's create yet another chunk. This one, the seat, I am going to animate to fly towards and past the camera to give the effect a bit more depth and make it look more dangerous.
Finally, I am also going to create one chunk for the tail and fling it off to the left side, rotating wildly, when the explosion hits. Of course how many chunks you want to create and how you want to animate them is entirely up to you and your personal taste and situation.
One important thing to do to make this look a little more realistic is to enable motion blur on all the bits and pieces of the motorcycle that are animated. You can do this by enabling the motion blur switch on the motorcycle chunk layers as well as on the composition.
Playing back our animation, the motorcycle should break apart and darken nicely just in time for our explosion.
Let's add that explosion so the motorcycle looks like it's not just falling apart due to old age!