Gearbox problems

[Rotorised]

I have recently pulled apart my gearbox to find that about %70 of all my internal gears have chipped teeth. The bike has always run fine and I always use the clutch on changes and changed the oil regularly with high quality oil.
Has anyone heard of this before? The first thing  I will do before putting it back together will be to sort out some sort of magnetic drain plug 
Also is there anyone here with any unused internal gears in good condition that they could part with for some cash?

Cheers
Brad

[hughes]

The ole 500 gear box is tough as nails. A photo of the gears could help some us tell maybe what went wrong, Sorry I have no gear sets. Most issue with the 500 gear box is shift forks bend.