Same thing happenned to my son's XR 80 a while back, and I was helped greatly by DRN. Throw a few chains and your case gets beat up. I used JB weld in stages to build up the areas taken out by the chain. The leak was small but real - puddle every day. Eventually I got the JB weld to fix it. Wet sandpaper smoothed it. Now it doesn't leak and you can't really tell there ever was a problem in the area.
To prevent this from happenning again, you must keep no more than 1 1/8' total chain slop, just like the chain protector sticker says, AND check your motor mount bolts regularly. This was the true source of the thrown chains, I believe. If the engine can flex a bit in the frame because of loose bolts, the alignment of the front sprocket gets off.
Thanks to DRN contributers, I was informed of this JB bond stuff and told to watch the motor mounts. This bike hasn't thrown a chain since.