Re: Choosing the right bike...and.....custom fuel tank
If you are building a frame mount kit and the price of the bike isn't your first consideration:
The first thing I look for in a bike to build a motorized bike from is front suspension. I have had bikes with and without it and I won't ride one without front suspension anymore. Cruisers with front suspension from the factory seem to be fairly rare. I-paint found one I think called a Kettler. I built my favorite bike out of a Felt cruiser but I added the front suspension.
The next thing I look for is a rear wheel without a coaster brake. I like good brakes. I don't like the coaster brakes much and they make it difficult to discard the poor/dangerous chain tensioners that come with the kits. If it has a rear derailleur then the tensioner won't be needed at all. If I plan to use a SBP shift-kit then one of the internally geared hubs are my preference.
The third thing I look for is Disc brakes. Rim brakes do work pretty good too. Just not as good in the wet.
Usually you can get some portion of these elements in a bike but not all of them. You then have to plan on whether or not to add the rest.