Post a picture. I'll add a picture of what I did if I can find one, but basically you drill holes along the crack about 3/8 - 1/2 inch from the crack on either side, then you use tie wraps to pull the crack together. It looks like Frankenstiens neck, but it works great.
You can try Plastic weld it looks like a soldering iron and there are plastic welding rods that go with it. bodyshops use it for repairing cracks in bumpers when people don't want to spend a ton of money fixing their cars. But doubt it will hold for long, of you can drill holes down each side of the crack and zip tie in an "XXXXXXXX" pattern looks a bit Frankenstein but it works me rear is like that. good luck.
yah cuz did a wheelie on his 660r crashed broke the finder off so we shaved both of them and one once missen a peice so we use plastic weld on it and it holds up and the shaved finders look good on his 660r
you can try the old school way..with zip ties...or do the rivet and tin method...with a piece of tin..big enough to cover the crack..drill some small holes with the drill bit the same size as the rivets..from under side..where the tin is..drill through that..through the plastic..place the rivet from the topside..down through the plastic..through the tin...crimp the rivet from the topside..while holding the tin from the other side..crimp it tight..this just leaves a small rivet head showing on the plastic..not as nearly ugly looking.< but..the zip tie does hold if your not worried about looks >..or..you can do what i do..i use some super glue..it's called gorilla glue..and..so far..it has been working really good on plastic..sooo..try which ever way you feal best at doing