How can you reproduce a machine part without the original drawing?
Start by defining the function and evidence, not by opening CAD. The physical part can provide geometry, but the manufacturing definition may also depend on mating components, material, fits, treatment, wear and operating conditions.
Step 1: search before measuring
Check maintenance archives, old supplier files, manuals, emails, machine backups and previous purchase records. Even an incomplete drawing can reveal nominal sizes or material information that the worn sample cannot.
Step 2: inspect the assembly context
Identify what the part locates, seals, transmits, supports or adjusts. Measure relevant mating features and separate functional surfaces from non-critical geometry.
Step 3: reconstruct and document uncertainty
Create CAD and drawings from evidence, but mark assumptions and dimensions that require verification. Do not hide uncertainty inside a precise-looking model.
Step 4: define inspection before manufacture
Decide which features must be measured or functionally checked so that the first manufactured part produces evidence for the next order.