I thought I'd share what I'm up to. I did a ton of research on bearing sizes, checking input shaft dimensions, spline counts on the diff, mounting locations, shifter compatibility on various transmissions and found that the manual trans in the 1G Insight, 1HCH and the GD3 Fit share so much that it parts should be interchangeable. I pulled a 1HCH 5spd from a junk yard and then swapped in the gears from an 07 Honda Fit. Only thing I have left to do is wait for the last shim that I need and then I can button it up and get it installed.
Yes, I know that means less MPG, but where I live and how I drive the shorter gearing is going to be so much more enjoyable.
Side note if I'm happy with the way things turn out I'll put up for sale the 1HCH guts. A person should be able to swap over the gears with Insight's 5spd. The only thing you can't use from the 1HCH is the differential, but the ring gear is the same dimensions and same bolt pattern, so can swap over as well if so desired.
The only really annoying thing is Honda hiked up the price on the tool for checking main shaft clearance. It delayed my progress for a month because when I ordered the part it was "back ordered" and they kept saying to check back in next week. I finally gave up on them and decided to look elsewhere and to my surprise everyone now had the tool listed for $200. So I ended up making my own for about $140. If someone has a good band saw and drill press they could make one for much much less. I can provide my cad drawings for those who want to make their own since it's still relevant for stock Insight trans.
Going through this whole process it really looks like Honda developed the 1HCH/Fit transmission and then shaved weight on anything they could for the 1G Insight. Pretty impressive with how much weight they took out of the transmission. And they even went pretty extreme with drilling holes in the rear drive gear and synchro hubs.