I would take it to a shop and see if they can get inside those small holes with a welding rod. The outside weld is easy access but that other it kind of tight. The problem I can foresee with getting the part put back in place is that you may not have a good solid corner to hold the head down and with the compression of a diesel you make get a leak you can't stop. If this is a gas block then perhaps you will get lucky and it will hold. You have to get the small holes sealed as those are the cooling parts of the water jacket that surrounds the piston. Only about 20 lbs of pressure there and no real problem with getting water into the oil should it not seal.
I would be looking for a new block as it may actually be cheaper than welding and doing the cylinder cleaning on the one you have.