Nice bike. I would like to find a nice 90s model sometime.
As for switching to a rear disk, I am not sure they had one. Having used both, a decent drum works as good as a disk. My 78k has a drum and it operates very well. I have good control and it only locks up if I want it to.
If you can find a model with dual disks, it is best just to swap the whole front end.
I wish I could get more info for you but not many here have the 90s models. So whatever info you can give us would be great. It is an area the is weak here and the more info we can get, the better.
The ride IS the adventure. The destination is just to get gas!