To repair cracks and chips in the interior ABS plastic (AKA "Royalite") trim, the Maintenance Manual gives a procedure that I've had good success with. The MM recommends using MEK (Methyl Ethyl Ketone) solvent as a plastic welding glue. MEK is available at larger hardware stores in the paint department.
The process is to paint MEK on the plastic edges and allow the solvent to soften the plastic, and then press the edges together and let the solvent flash off, leaving a joint that is nearly as strong as the original material. To fill small voids a paste of ABS plastic and MEK can be prepared and painted on in coats to fill the missing area. Small brushes are recommended. A metal or glass container to hold the solvent is needed, since it will attack most plastics.
You can obtain a pre-made ABS paste from Lowes and other hardware stores in the plumbing section. At Lowes it's in a blue can and it says "ABS Cement" on it. If you look at the ingredients on the label it will say that it is ABS and MEK. There are also several plastic pipe cements in the same area. Beware of any that list other types of plastic such as PVC or CPVC, as these will be just solvents with no ABS filler included. The pre-made ABS cement is best applied after a coat of MEK to soften the base plastic. This gets the ABS of the base part ready to fuse with the ABS in solution.
With a little patience and some scrap ABS sheet, doublers can be fabricated to glue on the back of cracked and chipped areas, especially helpful to back up screw holes that have cracked out or lost a pie wedge of plastic between the screw hole and the edge of the part. These doublers adhere very well if you paint the back of the part with MEK and wait for the plastic to soften, and paint the face of the doubler, and then re-wet the area on the part where you want to bond the doubler and press them together wet. Some Cleco clamps are helpful to hold the parts while they flash the solvent and solidify. Having the clamps lets you do several repairs without having to wait between each repair for the last one to become firm enough that it doesn't pop loose. Others have found that the cheap spring clamps from Harbor Freight or WalMart work well, with fairly large area, flat, self-aligning jaws.
ABS sheet and strips in small amounts can be obtained at your local hobby store - most carry the "Plastruct" brand (typical stock). Note that their styrene is white, and that the ABS is grey... the sales person might refer to it all as "plastic".
After a doubler is bonded on the back and cured, you can fill the area where the chunk was missing by painting coats on the front side. Just as with paint, several light coats is much better than fewer heavy coats, and you want to let them dry between coats. After 2 coats it probably will need to sit overnight to allow the solvent to evaporate from within the layers. If you have filled the area sufficiently, then you can sand the surface to smooth it and level it with the surrounding area. Old plastic being reused may benefit from a small doubler of thin 0.020 ABS sheet at each of the attaching screw holes to prevent new cracks.
On really big projects, especially external parts like elevator tips or tail cones, you might need to apply a larger doubler or hold a big crack in alignment. You can use the ABS just like aluminum sheet and make doublers to go behind cracks and hold the doublers with clecos in a series of rivet holes. The holes can be filled later and the part sanded smooth before painting. Carefully done, parts that would have seemed unsalvagable can be returned to service and look just like new after sanding smooth and repainting.
The interior plastic trim can be made to look like new with a coat of "vinyl dye". This is just a paint that is formulated to bond to the plastic. There are several brands, some available in rattle cans and some available in bulk for a spray gun. Automotive paint supply stores that serve the body shops will be able to mix the bulk type in a rainbow of colors, and usually stock SEM products. Many auto parts stores carry the rattle can variety in a half dozen colors. In preparation for applying the vinyl paint, all the dirt, oil, and contamination that has accumulated on 30 years of service needs to be removed. Begin with dishwashing soap and water, and use a scotch-brite pad to scour the surface. Rinse with running water and let it air dry. Just prior to spraying, a wipe down with enamel reducer or a proprietary wax and grease remover such as DuPont Prep-Sol using a clean white rag will help assure that the paint covers evenly and is able to get a good grip on the substrate.
If you wish to go an "All SEM" rattle-can route, the following products have worked well. SEM 38353 Plastic/Leather Prep is an Propane/Isobutane/Petroleum Solvent product which is useful for wiping down the plastic parts to remove any excess oils. Next, a combination of SEM 38363 Sand Free and SEM 15103 Super White Color Coat gives a original-looking finish. Sand Free is an adhesion promoter which softens the plastic (Acetone/Propane/Butane) and allows the Color Coat to "melt" into the plastic. Follow the instructions on the Sand Free can closely - and the results are amazing. (If you are doing extensive refinishing, you will need three or four cans of SEM Color Coat Super White, but the Sand Free and Plastic/Leather Prep go a long way, so multiple cans are not required.)