You would probably be safer using some inserts and copper olives and a compression joint, like the picture . However a “ modern day “ lol hep fitting should be ok . Personally I wouldn’t have any of that in my loft .
Remove what you need to back to a point don't mix fittings use the more modern white fittings and stainless inserts to make the joint to the existing grey pipe then use copper from there on .kop