Some people including me have repaired cracks by using tie wraps. Drill a hole at the very end of the crack to stop the crack, then drill holes on either side of the crack, and use tie wraps to pull the crack together, like Frankensteins neck.
A friend of mine used a product called Plastic Weld I forget who makes it but once he hit the trails it broke again with in minutes.
I used to work with Epoxy Resin and we used to use the kind that took 30 minutes to cure and came in 2 tubes resin A and B hardening and curing.
We tried that and lasted a few times out realistically a few hours each time but it did break again
They make a tool like an iron and it comes with plastic welding rods it's used in bodyshops to repair busted bumpers. That would be the only thing I think would come close to working, drilling and tie-wrapping is the best bet next to buying new plastics
i'm not sure how well you want to work on this..but..i have done it years ago..with some tin..some small sheet metal screws..and cordless drill and the right size drill bit..it worked for me..cut your tin big enough to cover your crack..pre-drill the tin..then from under the broken spot..place the tin over the crack..drill through your plastic..screw the sheet metal screws through the tin and into the plastic ( now your wondering what to do about the end of the screws sticking up ??..lol )..with a dremel and a cut off wheel..very slowly go to each screw tip...and slowly cut each tip off.. DO NOT FORCE OR GO FAST..or you will heat the screws up..and melt the plastic..if you go slow enough..one by one..you can cut them all off flush to the plastic..after that..just get some paint for plastic..touch up the screws where they stick through..all done !
I have an older honda ct90 that had cracked plastic body work. I fixed it by removing the part and cleaning the back (inside) surface with laquer thinner (acetone also works) sanding the back with 80 grit sand paper to rough the surface and give it (tooth) I then used whitewater (flexible) polyester resin although 2 part epoxy will also work as it has better adhesion, to make a patch. The way to make a patch is to cut a piece of fiberglass cloth to cover the break about 1 inch bigger than the break all the way around. Cut a second piece a half inch or so bigger than the first. Mix your resin and smear it around the break larger than the largest reinforcement. Stick on the small reinforcement add some more resin to make sure the cloth is saturated. Stick on the larger reinforcement add resin. Take a piece of saran wrap and smooth it over the patch to distribute the resin and smooth everything up. the repair should last you very good. Oh ya one more thing before starting with the resin stick some scotch tape over the crack on the (good) side to keep the resin off the finished surface.
Well I used plastic epoxy and it seemed to work until I went riding. It came apart and peeled off. So I used a hot glue gun and it worked so far after a 5 hour day of riding its still OK. Ill probably through some zip ties on it and re apply the hot glue. Eventually ill get a new fender but for now Im just letting it be.