I've used newer icomremote mics on older radios. Best bet is to find a newer one on someone's boat and borrow it for a test.
Also check the classifieds on CF for a used one. Someone had one for sale not long ago.
We had an HM-157B that worked with the M601, but the distributor made a small FoC mod to make it compatible. After 5 years the 157 is faulty, and I am looking for a replacement, but so far cannot find any details of the mod from ICOM. Anyone know of this mod?