Parts for Miroku Winchesters are hard to find. Shotgun Boogie Gun Works has springs for the rifles, but little else. Boogie might have some ideas about front sights; it's best to contact him.
At one time Winchester had sold a High Grade version of the 1873, with full 24" octagon barrel, checkered forearm and straight grip. I have one and I have used it on occasion. The Winchesters come with a slightly shorter lever throw than the Ubertis, but they have been short stroked, most notably by Cowboy Carty. Spare parts are difficult to find and the only source for springs that I know of is Shotgun Boogie.