XRAY - Model racing cars

After installing 300 motor I can achieve much bigger speeds. But while I braking car is turning 180 degree and big noise start to come from the diffs.

After two batteries I have opened the rear diff and find out that metal shaft not at the center of the gear and some teeth of gear is gone in very strange way - seems like pinon fired them. Later investigation shows that whole unit bends under stress loads and pinon start to scratch gear instead of being firmly connected.

What can be a solution? Ball diff?

Yes, ball diff. However, if you can find an older front diff (the one you tightened with a screw) this could be a temp solution