I have been riding motorcycles for 15 years in Japan and the US.
I have Yamaha Royal Star (1300cc V4) now.
http://www.liveinusa.net/moto/bike.htm
Please let me reply.
When I got motorcycle drivers license in Japan, it was hard to pass test.
Getting driver's license for big bike is easier now, though.
So, many person is riding big bike more than 1000cc.
I think you can sell your custom-built choppers in Japan, but you have to think about their vehicle inspection.
Their law for vehicle is so restricted. For example, you cannot attach anything pointy, must meet their very strict emission regulation, noise level from exhaust pipe, etc.
Good luck!