A part that is no longer available does not automatically make a device irreparable. Sometimes an equivalent can be found, sometimes a donor can be used and sometimes the part can be measured and made again.
Function matters more than outward appearance. Material, tolerances, temperature, electrical properties and load determine whether a reproduction is safe and durable.
Record dimensions, markings, function and environment. Search by manufacturer part number, generic specification and devices in which the same part is used. For electronics, datasheets and package or pinout information matter more than merely having the same shape.
When a specific part is no longer available new but appears in similar devices. Check wear and revision differences, however; a donor part may have the same age-related problems.
Yes, when its function and load are understood well enough. Plastic parts can sometimes be printed; metal parts can be turned, milled or made by hand; labels and panels can be redesigned. Safety-critical parts require additional restraint.
3D printing, Metalworking, Woodworking, Electronics and Basic repair can be combined depending on the part.