a furnace only has two parts - the blower system and the heating system. i was referring to the blower system as a whole - everything gone though (including controls) and aged parts were upgraded. the previous year I cleaned the burners and inspected them for cracks - all was well.
i bought a new motor because it wasn't worth replacing bearings in an old motor and yes, the newer motor is more efficient. a local motor rebuild shop quoted me $250 to rebuild the old motor and it would take a week. a week without heat in 20 deg temps wasn't acceptable. a new motor was available from the local Grangier the next morning. it was an easy decision. Since I was ordering the motor, a new fan also made sense. I cleaned the housing the motor and fan sit in and replaced the rubber grommets that improve vibration isolation. I also added damping to the housing to cut down on reverberation and went through most of the fasteners to ensure quiet operation. The blower is noticeably quieter now as well.
Reassembly still required careful attention to motor balance as you reassemble the fan on the shaft. Connections were not labeled on the control board as you would expect, so I had to understand what each wire was doing and where it needed to land based on the schematic for the control system.
if you "rebuild" an engine (something i've also done), that can still include new parts to replace old parts. it's the process of disassembly, diagnostics, and reassembly that defines it as a "rebuild". when i rebuilt a 4-bolt main 350 cu. in. engine, i only reused the block, heads, cam, and 4 of the pistons. two pistons, all of the valves, valve springs, intake, seals, carb, headers, etc. were new.