It's kind of tough to recommend a repair without knowing the cause of the cracks. They could be quench cracks from rapid cooling after the fire (water?). Rather than try to chase a lot of small cracks, it would probably be easier to cut out the area and weld in a piece of 5Cr plate rolled to the radius of the strainer. You should examine the weld preparation with liquid penetrant or magnetic particle examination after it is cut and beveled to ensure no cracks remain. If both sides of the weld are accessible, a double-vee groove should be used to minimize the welding, with root backouged to sound metal prior to welding the opposite side. A 400 F minimum preheat should be applied during welding and a 1300-1400 F posweld heat treatment will be needed. I believe the matching 5Cr electrode is E8015-B6 or E8018-B6. Perform MPI and hardness testing again after PWHT is complete.
Before cutting out a 200mm x 100mm area, I would excavate some of the cracks to see how deep they go. If they are not that deep, I think it would be easier to gouge or grind the surface until it is free of cracks, then repair with 8018-B6, and perform a stress relief. Do you think the part got above the lower transformation temperature during the fire to result in the hard areas? If so, the properties of the casting may be in doubt because the cooling rate could be just about anything. You have to get a good heat treatment in order to get the 90ksi minimum tensile strength specified for A217 C5 material. You can use 400F preheat minimum, but we have found 500F works better (fewer cracks).