Fairing repair with Plastic Weld and fiberglass
This is a fairly easy way to fix your broken plastics that some people may find easier and less smelly than the ABS cement method.
This technique uses a product named "Plastic Weld". It's marketed by a half-dozen different companies, but you should be able to walk into nearly any auto parts or hardware store and find something like this product from Permatex.
You can just smear this stuff all over something like a broken fairing corner (shown below) and stick it back together, but reinforcing the area would be better.
Reinforcement is as simple as using some fiberglass mat. This is a hole where the turn signal mount should have been. Rebuilding the area with Plastic Weld and fiberglass made it more durable with less flex than the other side:
If you're careful with the process, you can even manage to have the finished product turn out looking like there's just a small crack, or you can touch it up with some paint and have it look like it was a scratch. For instance, this fairing was in a bunch of pieces, with cracks all over the place. Now you can't tell that from more than a few feet away:
Clean the parts as well as possible with a degreaser. Then for cracks, use a dremel to put a very small notch (a V-shaped channel) on the back side. Into that put a filling of the Plastic Weld, then a layer of fiberglass over the crack. Make it an inch or two wider on either side for good coverage. Then work more Plastic Weld through the fiberglass from the inside until it is completely saturated. Working from the inside could save you from having to repaint the crack.
The technical bit behind this is that the fiberglass just adds strength in a way that only a mesh grid can, and the Plastic Weld acts as a bonding agent that sticks to the fairing and the fiberglass.
For basic cracks, it should be more than possible to carefully apply color-matched paint and have the fix all but vanish. You should only need to consider repainting for serious rash; simple tip-over cracks are easily and cheaply repairable.
To fill in actual holes or gaps, first do a layer or two as above, then cut up some of the fiberglass mat into small pieces (like, individual strands) and mix it up with the Plastic Weld. Then, fill in the holes/gaps with that substance. It's like the bondo filler stuff, only with the Plastic Weld as the bonding agent. Then just sand it down and do a little finish filling with normal body filler before sanding. Again, this is for gaps, where material is missing.
For big holes, such as repairing the usual turn-signal-punched-a-hole-in-the-fairing, You may want to do some additional reinforcement. Cut a template.
Aluminum is too heavy/bulky,
so use cardboard. That has the added advantage of soaking up the epoxy.
Work the Plastic Weld into the fiberglass from the back side. Use a plastic spreader (should be available at an auto parts store) to push the epoxy through the fiberglass so it's completely saturated. A small paintbrush may help as well.
Drill any holes that may be necessary.
Sand it down, apply regular body filler (there should be a kind for final 'finish' work) to the outside to smooth it out, then paint. You just saved yourself several hundred bucks.