I think that the market you are looking to create isn't one where you sell a new design retro motor, but where you sell rebuilt motors and take cores from your customers.
Harley guys have 1 engine. I know that there are shovels, knuckles, Eve's... All the same really. Vintage guys have multiple cylinder counts and the fashion flows a lot quicker. A few years ago there were so many more big multi cylinder cafe racers and brats. Now people seem drawn to scramblers with single or twin motors. A single motor in your case would be a liability as tastes evolve.
Sent from my iPad using Tapatalk