Is the 680 engine legal for DD?
 

If it is how much bhp should i expect over the standard and how much torque?

Yes.
class a is standard 618 or 583 overbored to 675 using the standard ducati bb kit
class b is standard 583

You get almost nothing extra from the 675 compared to the 583 cos the pistons are absolute shyte.
The 583 cylinder head has valves that are sized for a 350, and it will never develop serious power without valves the size of the 618/750 heads.

Doesn't the kit contain the heads and can shafts as well?

No, the 675 is barrels and pistons only.

Hmm .. there is another kit - for the 620 - cant remember the size, but I'm sure that is barrels and pistons too, and its not eligible for dd anyway.