A few thoughts, pardon if you've had 'em already
1: Check local laws regarding any
motorized bike, two or four stroke - doesn't matter.
2. Check local laws regarding becoming a motor vehicle dealer - particularly liability.
3. Build and sell a few low-cost examples before making any serious investment.
4. Don't get involved in the "used bike" game, vintage or otherwise it's done for love not profit - you'll never recoup your time and money.
5. This might sound odd - but you may not want to sell completed
motorized bikes at all for the following reasons;
A: They won't be broken in so odds are they won't run the best - if you break them in, they'll be used bikes by the time your done.
B: There's a inherent "Darwinism" involved with building a MB, people learn some of the shortcomings of these kits - if you just sell them a MB, you'll probably get swamped with "warranty" claims from those that don't understand it's not a motorcycle.
C: Related to the above - an inexpensive bike will be prone to additional problems, a quality bike would inflate the price to the point of excluding a large portion of those that would be interested in them.
I'd suggest simply selling kits and accessories, including a selection of bicycles ideal for motorizing, ranging in price from your typical $100 cruiser, a couple of "choppers", and a Worksman or two. Should they wish they can buy both the engine and a bike as a complete kit - or they can "contract" you to build it for them. That way you're "on the clock" should problems or the desire to customize arise.