The most important parts of a restoration are unseen. The bottom end, the crankshaft and rods with their associated shafts and bearings cannot be seen or measured without disassembly. Even new parts, installed by a fool, will soon break down. If you are buying an expensive bike, find out who did the bottom end. Was it some itinerant Harley guy outside of town, or a famous name like Maughan & Sons in England, Mike Parti, Big Sid, Haig Altounian, Mike White or Rip Treagle? Original parts are great for the exterior of your motorcycle, but the inner functional parts should have the upgrades that 40 or 50 years of advances in metallurgy and technology can provide. Why use Specialloid pistons on your Shadow with big clearances when the newer and tighter running ones are quieter and last longer? I say, inside the engine, it does not detract from the value to have repro parts. The possible exception would be where a very special engine in say, a famous racer still has the works-polished flywheels and rods, etc. If that's the case, keep these parts. On less special examples that will see some road use, go ahead and upgrade. The outside of the bike is another matter entirely. Currently, stainless steel fasteners and parts on Vincents don't detract from their value. Very few collectors want and are willing to keep up a bike with cadmium-plated parts. Even though cadmium is the original finish and a bike with these parts would be very unique, don't expect to see many. On the other hand, genuine alloy fenders are very rare on Vincents. These Birmabrite alloy pieces are almost all thoroughly dented now, and a dent stretches the metal, making the dents very difficult to remove. Stainless fenders run a distant second place in my opinion.