No problem Tony,
Sorry I probably overreacted; there's been far too many amateurs on here recently seeking help for their daily work, for which they are clearly unqualified.
My first approach would be to find out what is standard in your industry, starting with your the supplier (probably a jobber, so maybe no help at all). Google might give up information if you are persistent and creative in your search terms.
If starting from first principles, I would follow Code logic, which is that the rated tensile strength of the filler metal should match or exceed that of the base metal, in the same condition (i.e., as-welded or PWHT). K55 has a UTS greater than 70ksi, so you are correct, standard 70ksi class fillers are inadequate. ER80S-D2 is a popular low alloy filler that also has very good impact properties, but I don't know if it will match the strength of K55.
There also exists the classification ER90S-D2 with the same composition, but AWS rules suggest it is used with an argon mix; i.e., not suitable for GTAW process. In the end you may end up going to a -G suffix classification, which compositions vary according to manufacturer ('G' is basically a wild card). At this point you need to obtain advice from the consumable maker (Lincoln, Arcos, Hobart, etc).
I'll be interested to know what you find.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."