[beatr911]

So building one of these will most likely require cutting, welding and fabbing the rear frame section, sacrificing the original bike. The bolt on rear subframe bikes like the WR will be exempt for the most part. To make room for the rider the swingarm probably will need extending at least 4", which is commonly done on a range of bikes so is probably not a big deal especially if working with steel vs. aluminum.

Ergonomics:
Humans need small bike cross sections directly in front of the crotch in order to fit without splaying knees out in the wind. When seated just above the swingarm the rear loop of the frame is directy in front of the rider. Perimeter style frames are widest at this point so it will require modification or a narrow section here.

Hands and feet when seated in a semi-reclined position typically require that the feet be forward of the hands to reduce knee angle and reduce reach to the bars. Forward foot controls and rearward handlebar placement is then desired. More fabbing to place foot placement as far forward as possible without interfering with the front wheel. Additionally, a "tiller" style bar to reduce reach.

Electrics: The battery on most bikes is directly under the seat. The open space between the (now forward) footpegs is a good relocation area. It also helps keep the center of gravity low.

Air box: This also is usually under the seat. Hmmm, maybe a short cone filter or piping the intake to another location will work.

Fuel tank: This is tougher if it is to be relocated. Leaving the donor bike tank in place keeps the CG high but may be worth it. Fabbing a tank to fit into the cavity created by the extended swing arm could be a solution also helping keep CG low.

Rear suspension: A single shock system is desirable as it is compact and doesn't require an especially strong rear subframe, easing fab requirements. Dual shock systems reduce swingarm loads, easing extension fab requirements and open up the center of the bike for other uses like a fuel tank, air box or battery.

Donor bikes I'm looking at are 250cc as they have outstanding MPG potential and really need aerodynamic drag reduction to do well at higher speeds. Cheap is also good, so finding a lightly damaged bike is a possible route.

250 Cruiser bikes already have a low seat height, small diameter wheels (lower swing arm height) are pretty common and cheap.

The Ninja 250 is really common, may be even cheaper but bars and frame mods may be more extensive. Attached are some pix of a mod started by an ecomodder that wasn't finished (search for it).