Hmm, it may take a bit of work to get them looking like that as they are a completely different drawing style. You may need to redraw them using simpler shapes. However, you could try a couple of things. First of all use the Posterize effect to reduce the amount of Colors used. To give it a more "blocky" look. In the comp panel, go into the Resolution menu and change it to "Custom". Experiment by manually adjusting the numbers. This allows you to reduce the actual display resolution, so you can create the effect of these old computer games. However, the characters in those games were actually designed with the resolution in mind so yours may not work quite so well.
Anyway, once you've done this, in order to render it out this way, you need to make sure to render at current resolution. You also need to then go into the Output Module settings and use the Stretch options to force the comp size back to it's original size.
There is also the Mosaic effect but I never fins it looks as authentic as actually messing with the resolution settings.
Angie Taylor animation & illustration for television, film, web and devices