Although the specific KNS-80 and KNS-81 installation manuals only specify certain King indicators as compatible, there are many different indicators that can be used with these navigation systems. The King installation manuals indicate the following nav indicators as compatible:

KI-206
KNI-520
KPI-552
KPI-553

(note: the manual does list some much older indicators as well which are obsolete and unsupportable)

In addition, the following indicators will interface and provide the necessary functions for VOR/LOC/GS navigation:

KI-202   (VOR/LOC only)
IND-350   (VOR/LOC only)
IND-350A   (VOR/LOC only)
IND-351
IND-351A
IND-351D

More or less, any "non-converter" indicator should function properly with the KNS-80 and KNS-81. Please note that it is ultimately the responsibility of the installer to verify system performance, functionality, and airworthiness.