Ok, if it were me I would not consider it unless this was my very last option (scrap the vessel). First, you should not be mixing code of construction rules. This is my opinion, and if I were the owner either you repair/modify to the original rules or you would have to jump through many large engineering analysis hoops to satisfy me that the code rules are similar between BS5500 and ASME B&PV Code, Section VIII, Div 1.