All the Alto schematics are available at Al Kossow's Bitsavers [1]. If you want to understand how it works, start with the hardware manual [2]. The Alto processor could probably be kept running indefinitely, since it uses standard parts that can be replaced. The monitor is more difficult; the Living Computer Museum made some new monitor boards for their Altos, cloning the existing boards. The disk drives are another potential maintenance nightmare; I wonder if it would be possible to build a flash-based disk emulator.
Making a FPGA-based Alto clone would be a possibility.
LCM built MASSBUS adapters for their PDP-10's, its probably possible. (to be clear, the ALTO was built in total from COTS parts, the disks are of a known type used on many other minicomputers, including I believe the DG NOVA)
Making a FPGA-based Alto clone would be a possibility.
[1] http://bitsavers.org/pdf/xerox/alto/schematics
[2] http://bitsavers.org/pdf/xerox/alto/AltoHWRef.part1.pdf