It really depends on the equipment you have available. With the best tools and such replicating a part is very easy. Doing it by hand the old fashion way can be much more complicated.
Here is Jay Leno talking about the replication of old car parts using a 3D laser scanner and a 3D printer. Printing parts in plastic after taking a scan of a part.
The image they make can be easily put into a CNC mill and milled out of a block of metal.
Its pretty cool technology. The laser scanner will become standard equipment for CNC machinist in this decade.
Laser scanner + CNC = replicate just about any metal part.