Replacing the stock and forend sounds rather expensive.
Does it have a doll's head extension or Greener crossbolt?
What is the pin/screw configuration at the back of the box?
Any sling swivels, cheekpiece, horn pieces or menacing muzzle?
What grade of Krupp steel tubes? Sauer was big buddies with Krupp and purported as the largest purchaser of Krupp steel for tubes; therefore, it could be possibly that most, if not all, Krupp tubes with the Krupp Lauf Stahl, 3 rings, etc. was sourced from Sauer as were many frames.
Does it have the large and/or small crowns on the barrel flats?

Sounds like a S,D&G from 1910, 1911, 1912 as a guess.

Sauer No. 1 - doll's head, upper hinged/overhanging scear

Sauer No. 2 - Greener crossbolt, upper hinged and lower intercepting scears

Sauer No. 3 - same as No. 2 with sideclips & select walnut and engraving.

I doubt it to be a 40, 45 or 60.

Procure a 1907 or 1912 S,D&G reprint from GGCA to go along with it.

Kind Regards,

Raimey
rse