These bikes are too heavy to make an effective cafe out of. They're made for long hauls power and comfort. Better to restore to what was intended and buy a smaller bike to turn into a tight twisty street fighter. Then you'll have the best of both worlds.
Yes, definitely change the rear gear case lube and service the drive shaft. It is wise to perform all of the routine maintenance since you have no idea if any of it was ever done.
On my Concours, I changed the gear case oil and did the drive shaft maintenance every time I changed the rear tire. That was more frequently than the service manual calls for, but it was easier to do them all at the same time.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough