I have had to repair the front and rear cowl due to some crash damage. These are made from ABS plastic. The side fairings are made of Poly-Propylene of some variety, and will not work as well with this method.
The steps I took were:

• holding the crack together as close as possible to original with a vice grip
• clean with prepsol
• mark the crack to make it easy to see
• chamfer the inside of the crack into a "V"
• clean with prepsol
• glue
• chamfer the outside of the crack into a "V"
• clean with prepsol
• glue
• sand
• paint

caveat: I am not a smash repairer, just a dude.

le crack

clean with prep sol

clamp it as well as you can and mark it

I used a die grinder to grind a V, a dremel would work nicely as well.

60ml of ABS Cement. It is ABS in a solvent. The solvent melts the ABS then evaporates leaving nothing but ABS. nice.

I have come to the end of my run of five uploads. I'm sorry about the quality but they have gone from my phone, then scaled to fit the upload, then saved as ~.png.

 

Part B.
A piccy after the V is ground, don't forget to rub some prepsol in there.

The last dodgy shot with the bead of glue in there

I did this in three stages, Inside, around the lip where the clamp is, then the outside. I let it dry overnight after each stage.
I have given it a sand with 120 grit, then 400 grit, and given it a coat of primer. I have to fill some holes and improve my sanding and give it another coat of primer, but that will be tomorrow.

I got the glue from a shop that sells plastic repair kits to the Smash repairers. The plastic repair kit costs around AUS$1000 as it comes with a welding tool, plastic rods, powders in various colours, droppers etc. I just bought the replacement bottle for about $12.

 

Here is a moderately better shot of the cowl after the glue.

After I painted it with matt black (man's colour), the repair was still visible and had some pin holes and sat a bit proud of the surface in one spot. I filled the pin holes with "Stop putty".