Radio "Hash"

I've been here before with a similar question, but maybe I can be a
little more specific now. I have a vintage 1972 aircraft with a Lyc
O-360 (McCulloch J-2). When I bought it, she had the original Bendix
radio in it. I replaced it with an ICOM A-200 and a brand new Comant
antenna, or rather a radio guy did. He can't figure out why there is a
constant hash noise. I took her in for my annual, and the A&P said
that the old Bendix mags do not have shielded wiring, and that's the
cause of the noise. I didn't go any further with it, as I now fly out
of a little private strip and can turn the radio down after I leave the
pattern. But I did have to talk to Charlotte approach the other day,
so this thing has to be resolved.

Is it an easy task to replace the unshielded wiring on the old Bendix
mags? I hate to replace the mags, as they're working great. They have
510 hours on them. I used to get the hash on primarily one mag, but
now it's equal on each. Am I correct in assuming that the hash gets
into the radio from the pos. input, or can the ground contribute?
Would wiring the radio power directly to the pos anode of the battery
with a circuit breaker inline, and running a separate ground for the
radio help, or am I just whistling Dixie? I did solve the problem of
an erratic volt-meter in another plane by giving it it's own ground.
Thanks for any help and suggestions.