Not sure why no one seems to be forthcomming with battery info!
Yuasa may be able to help with the battery size designation, they will want exact model and year info, try them at:
http://www.yuasabatteries.com/contactus.phpI don't currently have a 90, but did years ago. As I remember, battery goes under the right side cover, held in place by a rubber strap. I think the correct battery comes with a red and black lead which plug into matching wires in the frame harness. Many bikes get modified over the years so it's hard to be sure what you are looking at. If you can post a pic of what you've got, perhaps someone more knowlegable can point out what's what on your bike.
I'm sure someone here can come up with the info you need!