Several Sansui models as well as Pioneer use the dual jFET 2SK129A, which are practically unobtanium and at their age, getting unreliable at best.
I have discovered after experimenting with an AU-519 that the modern 2SK2145BL is an ideal replacement. No rebiasing ot twaddling with component values are required (for the Sansui flat-amp board, anyway)
Only problem is it seems only available as SMD. So, tack-solder wires to the leads and dip generously in twin-tube epoxy to make a "case" for it, as the SMD leads are extremely fragile.
I actually molded a case by taking a small pushbutton cover, lining it with wax paper, filling it with epoxy, putting the SMD with leads attached in it and let it set overnight.
From Googling, it seems replacing this old transistor is a bane to many and has resulted in a lot of good receivers ending up in the trash bin
Hope this info is of use.