The leak looks like it is between the body and the rusty screw. I would try to find stainless steel (SS) or brass screws/bolts that are exactly the same size (thread, length, width and head) as the existing, replace all screws. Also replace the washers with SS, brass or nylon washers. Rusty washers and bolts rarely seal well. I didn't see any cracks on the housing, but you must inspect it closely. The housing surface, where the washer sits and both sides of the washers, need to be perfectly flat, clean, dry and smooth, no scratches. Some 3M silicon(or other very high quality) for wet areas or gasket silicon/sealer on both sides of the washer, would help seal the leak. The sealers must be applied to clean dry surfaces and must be totally dry before you test. I use silicon or plumbers putty. Read the instructions on the sealers and give extra time for curing.
Finding the parts maybe challenging but I would contact Hitachi, at their Bangkok offices. If the original parts are not rust proof, try to find the SS, or brass ones. Bangkok will have them, maybe near China town.