Walnut has a density of about .6 gm/cc.
Micarta has a density of 1.3 gm/cc or higher, depending on the fill material. So a micarta stock would weigh at least twice as much as a walnut stock.
If you are going to be removing material from any kind of stock it will have to be from the inside. Otherwise you'll end up with a tiny tiny stock !! If I remember right the Remington nylon 66 had a hollow stock. Should be at least as strong as a plastic canoe.