I had an instrument panel on a Cherokee that was broken, pieces missing, cracks etc. I bought the plastic repair kit from Plastex and repaired it using their recommendations, re painted and had a nice looking panel. Seems this stuff works like plastic welding and is sandable, paintable and moldable. I have used it on plastic wing tips, rudder tips and all plastic parts with good results. My A&P had never heard of the stuff, now he has two kits in the shop and uses them regularly.
http://www.plastex.net