The issue isn't necessarily whether some guy in his workshop can make something work, it can be who is liable (and for what) when it doesn't and an accident happens.

An engineer told me that if one uses manufacturer-designed parts in the way that they were designed and follow manufacturer's repair standards, then the design liability is on the manufacturer. If, however, one uses a "creative solution" because parts are no longer available, the person doing the work (or his employer) can be liable for the unproven design.