Over the years, I've restored a few stock bikes plus built 3 one-off type bikes. For the stock bikes it's been straightforward as the design is already done and generally all of the new parts needed are made by other people, I just bolt them on or fit them carefully.
We know exactly how the bike goes together and what the finished product should look like. There are parts books, shop manuals, repair or restoration manuals and even DVDs to help.
For the custom items on the one-off, there is none of that. You are on your own with your imagination as the main tool.
The one off bikes definitely require a lot more head scratching. I end up making a lot of parts as the part I want or need often doesn't exist. First it has to be designed, then it has to be made. It takes a few tries to get it right sometimes.
It's a treat to do a stock bike afterward, but the challenge of the one-off is also rewarding.
Glen