To repair whole area of bad DIY checkering calls for a high level of skill to the point that if you are that good it would be quicker if the gun is a box lock to replace the stock.
Small damaged areas can be repaired by mixing fine wood dust with glue and using this as a filling, when dry it can be re-checkered even this has a major draw back after you re cut the checkering the glue component stops any way of colouring the repair with stain. so it is colour the wood mix first and hope it blends in.
I have replaced a major area of chequering only once and to be honest it was an experience I would not like to repeat. Having said that the finished look was far better than I expected.
It sounds simple but it is difficult to achieve. First you shave what is left of the damaged checkering down to un-checkered timber. Next you cut a thick veneer of walnut about 1/8 inch thick and shaping it to fit the cut out area you have made. Now steam your patch until it becomes extremely pliable apply a water based Epoxy Adhesive like "Cascamite", work quickly positioning the wood patch then bind it tightly in place using strips of bicycle tyre inertube, this new wood can be shaped re-checkered and coloured to match.
A lot of effort for such little return, but if the original stock is a piece of rich dark and beautiful Turkish Walnut it is worth a try.
