If I'm not wrong then Fregat are simple surface search radar that provides targetting data to the Shtil "system", they themselves are not targetting radar.
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So as long as you have those 3K92E1 radar illuminators you can use very much any good 3D radar to fire Shtil missiles.
Coming to VL-SRSAM, it's way way more advanced than Shtil. One, it has onboard datalink and probably the most important, it's active radar homing not semi-active like Shtil or Buk where you need an illuminator to constantly focus on a target.
We can use virtually any radar to guide the missile using datalink and then switching to active-seeking as soon as its onboard radar seeker detects the target.
Edit : You asked about Fregat's replacement too. Sorry, missed that
We recently signed for Lanza-N radar, those can pretty well be the replacement. Rather they're better in almost all criteria, from range to weight