did it stop working or what

looks like diff has gone and shot a bit through the box

Did it break out, or in?
If the broken piece was hit outwards, then as Dave says, something on the diff has come off and been punched through the casing. There are previous issues on here where the bolts holding the crown wheel on have come loose and done this.

Options:
Replace gearbox. Take care to get the right one to match the original.
Strip gearbox to inspect for cause and other damage. Weld up casing and replace displaced bolt. Rebuild. May be lucky.

If the break was from the outside, then something in the clutch housing has been smacked against the casing. Cause should be more obvious.

I don't think it's smart to use it again. Because there was no oil in there for some time i think. And in 5th and 3th it plops out! So i want to inspect The gears after new years. Maybe it can be used again. For now got a gear box from my old engine a 8v instad 16v but it drives vine now. Because i got a box with The engine. But nobody says sometings wrong with it.

Daves' bang on with this one, I'd say the diff pin has shot out.