the Insight battery pack is about the size of a small suit case, and I suspect weighs about 60-70 pounds at the most, and Honda does not sell them, they do however replace them when they wear out, but they will not sell that part to you.
if you want to swap engines, the Fit suposedly had the same engine mounts(???) or something simaler to that, and if you go out side the US you can get a Fit with the Dual Point Ignition system, that is suposed to burn closer to 100% of the fuel, and burn it faster, giving you a cleaner burn, and more power by useing all of the fuel, a nice efficent engine design, better then V-Tec for efficentcy, but lower power.
I wouldn't call its aerodynamics world class. Sure, it's the most aerodynamic car the automakers have been willing to let us buy in recent years, but cars like the Tatra T77a(.21 Cd), Alfa Romeo BAT7(.19 Cd), Ford Probe V(.14 Cd) and others had it beat decades before the first model ever rolled off the assembly line.
The aero-design of the Insight is on the fringe of general public "Acceptance" without ridicule -- anything more, and the public runs away. IMO, I think it's a great compromise of this acceptance, functionality, and FE. Some people still have a problem with it, but so be it.
All I know is that I once drove a 5-speed model around a small town, and honestly loved every minute of it. I hated to give it back!
The point being, a transplant into this platform sounds like a fantastic idea. For example, after one is purchased well-used, and the batts cease to function. It's mass-produced, has the obvious FE goodies minus the drivetrain, and well, is a Honda I'd love to see the results of such a venture.