I am not really sure this is easily done in anim8or, but I can show you how I would do it knowing what I do know... Yes, it is a lot of work, but if you really want the effect, as I did with my example, you do what you have to do, knowing what you know:
1: Choose the base color that you want your engine-bright to be... and then apply it to that engine area of your ship. For the sake of discussion, make it light blue., very bright.
2: render your scene frame-by-frame in BMP or JPG.
3: Open your choice of image editor... I use GIMP, but GIMP or Photoshop or similar program...
4: Open your BMP or JPG movie frames one at a time... and select the light blue areas you want to glow...
5: Make those engine glow areas into a new layer on top of the original frame.
6: Blur your engine glow layer to the desired glow effectiveness.
7: Save your image in its original BMP or JPG format, knowing that the layers will merge back into one, this time with the glow/blur atop your original flying ship image.
8: Repeat this process for each frame.
Then, combine the edited, now-glowing frames into your intended video format (avi?).
You will have glowing engines.
I did a similar thing when I wanted a glowing effect in a space thing...
If you open the below GIF, you should see a stars in space scene that has 3 faint blue glows come together, brighten and then materialize into a triangle form of three distinct dots with a small "ship" in the middle.
The idea was to have a mass of energy materialize, coalesce and condense into the three spots of light, a ship appearing out of nowhere as if materializing from a wormhole. From this point the ship would then drift steadily toward the camera and then on to its destination.
The difference between my attached file effect and your intended effect is that my "light" starts extremely faint and large, and then shrinks in size and lessens in blur, giving the effect of the energy condensing and increasing in intensity, but then fading as it dissipates, leaving in its place the newly appeared spacecraft.
Your ship glows would blur, but as the ships become smaller, more distant, you would scale down the blur to match the idea of their becoming further away.
Does that make sense? Or would you like me to take time to make a sort of pictorial tutorial?