Cut the side out of a suitable sized plastic bottle then pop rivet it inside the damaged guard in a fashion wich allows it to create a 'beak' which roughly replicated the length and curve of the original guard. Heavily abrade the surface of the mudguard for about 6 inches back into sound material (I would use a grinder with a 40 grit flap wheel). Form if needs be a plaster of paris 'mould' over the plastic bottle to get the internal curvature correct and once happy cover in cling film. Mix some fibre glass resin and hardener then lay alternate crisscrossing sheets of fibre matting and resin making sure it overlaps well into the abraded areaat least 3 or 4 inches....allow to cure. Carefully remove the mould and plastic bottle former and turn the mudguard over. Abrade the inner in the same fashion and lay up fibreglass in the same fashion as before but this time inside the guard..leave to cure. Once set up grind, sand , generally trim and fashion to as near the correct shape as possible. Use car body filler to take out any imperfections, flat, prime, flat ...basecoat...then clear. Spray inside matt black polish exterior with T Cut then wax...fit mudguard to bike....Long winded and labour intesnsive...but not that dear and there is a perverse satisfaction in figuring out how to do these things...alternatively take the broken bits of guard along to an intelligent car/bike repairs place and they may be able to plastic weld and spray it for you.