I have heard of guys taking a bike apart and shipping pieces of it in 5 or 6 boxes as motorcycle parts, only to get the bike into the country where the parts are then assembled into the original motorcycle. This was done by a dealer here in Colorado as well as a collector I know.
It sound extreme but it is a way to by pass the motorcycle custom problems. the question however is how many boxes and over how much time to you have the bike shipped from over seas a little bit at a time.
Of course once the parts are reassembled into a real motorcycle again you may have problems getting it registered, and/or titled, but each state is different.
In Colorado for example, I have had no problem having MotoCross/Cross/MX bikes made street legal. I built a frame up bike using a NOS ( New Old Stock) Rickman rolling chassis mated to a Yamaha road racing engine project bike I built and got it registered , plated and titled here in Colorado with only minimal difficulties.
But Colorado DMV laws are not to strict and seem extremely accepting to getting motor vehicles and home made project vehicles onto the roads.
What BIMOTAs are you looking for??? You might find some one here on this website with one for sale if you post what you are looking for.
Good luck.....