A slightly larger bolt head and maybe a mod to the mag box and a new bbl are are about all you need..
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(Note, all of my measurements were taken with my rifle and with rounds I had around.)
The existing bolt head is 0.655" wide. The width of the part of the bolt that's around the rim of the brass is 0.035". The rim on the 7.62.x54R brass is 0.579" wide, and the rim on the 45/70 brass is 0.605" wide. You'd have to open-up the bolt by about 0.013" all of the way around. That would leave only 0.022" of thickness on the bolt head around the rim of the brass. There might be enough steel left. The rim on a 45/70 is also thicker, so you'd probably have to do some work on the extractor. Tapco carries the bolt heads for only $9.99 each, so you could afford to practise.
Converting the magazine looks doable. You'd also have to open-up the bottom of the receiver a little. I tried to cycle some 45/70 rounds with the magazine removed, and it looked like a few minutes with a Dremel would be all that you would need. The magwell narrows towards the front, so you would have to expand the sheetmetal magwell. A flat surface and a big hammer device (BHD) might be all you need. Tapco carries them for $7.99 each or $15 for ten, so you could afford to make a few mistakes. You'd also have to do some Dremel work on the stock to get the expanded magwell to fit. The magwell probably wouldn't look nice, but I'm sure it could be made to work.
Feeding the round into the chamber seems like, other than actually finding and installing the barrel, it would be the hard part. You would have to come-up with some way to get the 45/70 round aimed just right to get it to fit into the chamber. Even with rounds using pointed tips, I couldn't get it to feed. The edge of the chamber dented the lead.z