Random assortment of comments:
A pistol grip stock will fit a straight grip receiver, but with a gap on the bottom. The gap could be filled with epoxy. A gunsmith or welder could modify the lever to a matching curve.
Marlin made the 336 in 44 Magnum before they re-introduced the Model 1894. You could use or copy those internal parts.
The 444 is not just a longer 44 magnum. The 44 is straight cased, the 444 is mildly tapered, with a larger head, though the rims are the same, and both approximately the same as the 30-30 rim. Diagrams at: