the dimensions of my battery support brackets and recess are no where near what reed quoted. Thats like a conventional shaped rectangular battery.
On my '65 Sport 90, I checked and double checked my dimensions. They came out right around 2 3/8" (wide) x 2 1/4" (long) x 5 1/8" Tall). Basically a square base battery that is abnormally tall. I just ordered a 6N4A-4D battery which matched these dimensions very closely at 2.36" x 2.2" x 5.12".
In another thread, they describe the battery as a 6n4a-4b, which I could not find anywhere. I got tired of searching for this stuff online because their electronic catalog applications start out around 1980. SO I had to go to shop to get an actual catalog with dimensions. Thats where I saw the '4D' version was a good fit. Once I had a good part number, then I could just search on that, void of any particular application. At least thats the story for now. I will update when it arrives and I see how it fits. dimensionally, it should be perfect and sit in the cavity and be supported properly.
I am uncertain if different 90cc models came with different batteries. Like I said mine is a sport 90. However, your description sounds more like the battery mine measured out to.