I believe the targa newfoundland rules state that turbine casings and exhaust casings shall be OEM, in Level 2 preparation. Then they go on to say that the internals are free. By this definition, any CT26 hybrid of any combination meets the specifications of "OEM". So if you're asking if it meets the rules of a particular event, posting the rules would help.
I think it's easier to call a horse a horse, and a 57 trim CT26 is not "stock", it's a 57 trim CT26. If you're asking if you can say to others it's "stock", when you whip them good, then sure, because I do ;D .... in all seriousness, it's easier than explaining the principles of rebuilding a turbo. Obviously, a simply rebuilt CT26 would qualify as OEM under any rules structure. Just like a rebuilt starter, alt, or engine block, it's the same as it would be from the dealership. Adding different compressor or turbine wheels to the OEM housings now puts you in the grey area where interpretation is paramount.