Hi do you think this casing could be repaired, its the magneto cover from a dt 250, also the sprocket guard, they seem pretty rare to get hold of.

I'd keep looking as i guess its the same cover for a lot of years with that same shape motor--even off a 400 is likely the same. Check Partzilla although only takes a minor irrelevant change to alter the part number--even color change.

Welding will be impossible, as it´s Magnesium and the broken parts are missing.
The round cover is not rare, you can find good used ones. The sloted cover is more difficult to get, most of them are broken from vibrations.
If there is a used one in acceptable condition, you can support or fix it with Epoxy and glass fibres.
See attached