Magazines to suit 45-70 and 45-90 magazines are still available, I recently purchased one from an Australian gunshop that occasionally advertises them on Used guns.com.
I'm going thru the same thought process, I recently purchased a "new old Stock" No1 mk111 receiver in the white, brand new , never used, not even a serial number stamped. and I'm thinking about what to build it into, I have the bits to turn it into a nice .303 sporter, but a 45-70 would be interesting. my only concern is the machining that will have to be done to the receiver to fit the larger diameter cartridge. A local lee Enfield amateur gunsmith maintains that instead of machining the receiver, the 45-70 rim can be machined back ( by trial and error , I guess) to a diam the feeds without touching the receiver. food for thought.