Hi DUX,
I'm in the process of repairing a MC18 fairing and i'm using some ABS welding sticks and a hot air welder to repair the cracks.
To restore the tabs or refill gaps the i'm using a 2 component clay to mold the shape required and then i melt the sticks in to the mold. Once the tab is molded to fit in place i weld it to ther fairing again using the hot air welder, then with a little file i remove the waste so it adjust to the original shape.
As the whole fairning needed some restore i'm taking more time than expected, but if you want i can make a little description of the process with some pics to help.
I also bought a kit of plastex repair kit, it seems to work great but i haven't tried yet.
If you don't haves access to a hot air welder you can use a meddium/large soldering iron to weld the plastic.
Javi.