I've never heard of breaking off the trim with pliers. It might work or it might go terribly wrong, so I'd pay a gunsmith to do it. Any local gunsmith should be able to accomplish this simple modification. Just touch-up reblue the receiver afterwards. You could do this yourself with a Dremil if you have practiced cutting metal with one before.
An AK grip will fit inside the trim on the rifle as it is, if you file/grind/carve some of the grip material away. The AK grip screw is longer and of the same thread pattern as the one on the CZ858, but the grip nuts are not interchangeable because the AK grip nut is too large to fit through your receiver's square hole. Some plastic AK grips use a short bolt, such as the Hogue AK Rubber grip, to which I added a 'star' lock washer to hold it in better.
I don't like the original CZ858 grip because it is too short and the wrong angle. It causes my pinkie finger to slip off and dangle, while the metal trim digs into the web of my hand between the thumb and index finger. So I put an Israeli plastic grip on my rifle. It has the correct forward angle to avoid the trim digging into my hand. I replaced the short metric hex head screw with a phillips head screw from the hardware store to simplify possible field repairs.
So look for a good AK grip that fits your hand well but is not too thick on the top, so you won’t have to carve it up too much to fit it. The Tapco SAW grip looks like it has the correct angle but I’ve never tried it. I would avoid using a regular stock AK grip if possible, as they are too thin and short for many people and you have numerous other options available.
