Vintage Synth Repair

I have repaired my Arp Solus.  To review, the Solus had a weak output
with very apparent popping sound at key press and release.  Also, when
the LFO was enabled to control both FM and PWM, the synth would start
some mystery oscillation, the rate of which could be controlled by the
Attack and Release sliders!  After ruling out other suspects (key
contacts, trigger circuit, envelope circuit), I traced the problem
back to the VCA, specifically a 4558 8-pin IC labeled Z20 in the
schematic.  

In addition, an "excellent, working Octave Cat" I recently purchased
arrived with a broken switch, a malfunctioning VCO, and CV tracking
problems (don't buy from donesha2007 on ebay).  The fix for the CV
problem was also replacement of a 4558 IC (socketed components so it's
pretty easy).

So be suspicious of those pesky 4558's!  Is there something comparable
of better quality?  Anyway, I hope this is useful to someone.

--Stephen