I can't see the point.
An '86 level smokeless .45-70 will do nearly anything a .45-90 ever would.
Even in black, I recall the old advertisement for the 330 gr Gould Express .45-70 bullet: "This bullet, loaded with Hazard's Ducking Powder, will shoot practically as flat as a .45-90 and with better accuracy."
Then you talk about going back and shooting .45-70 in it after rechambering. That was an improvisation back when you could not get any of the old cartridge or brass at all. If you want a .45-90, load .45-90 for it. If you want to shoot .45-70, leave the rifle alone.
I presume it is one of the current Jap copies and not a real extra light weight Winchester for which rechambering would be butchery.