I've never seen a "glue" or "heat" type repair made for the high-density, impact resistant plastics used in snowmachine cowlings, motor cycles etc. and JD tractors.
The repairs I have noticed typically involves some manner of sewing or bracing the two edges together with small bolts or screws and metal brackets.
Drilling small holes and heavy monofilament line and sewing the edges together works quite well. It ain't very pretty --- but it does the job!
If it's at the attachment point to the ROPS; maybe you can fabricate some brackets to hold the fender to the post. Likely gonna look a bit Frankensteinish but it would save you the $$$ of a new one.
AKfish