1981 csr six fifty ltd brake res

Trying to help a bud rebuild his bike and he needs the rear brake reservoir and brake line to the caliper. Can't find info on the 81...will an 82 0r 83 work???

The 81-83 KZ650-CSRs came stock w a rear drum brake.

Maybe it's a 750 ltd, I'll have to check.

The 1980 650-Ltd also has a rear drum.
If the bike is a 750-Ltd-4, then the 81-83 rear brake parts should interchange. According to the diagrams at Kawasaki.com, the same rear caliper, disk & master cylinder are used on those models as well as the 750-Standard-4s & a couple of other models, depending upon the part.