If at all possible, try to save the original stock.
Marlin does, or did, final fit the stocks by heating the receiver tangs and pressing the stock on.
If you remove the stock you'll see scorched areas.
This process insured one of the tightest wood to metal fits in the industry.
A replacement stock won't fit as well, will be rather expensive, and won't be original.
A crack can be repaired by a number of methods, including using epoxy or a special "Crazy Glue" type wood glue that quickly soaks into even fine cracks.
The Marlin 39-A is the finest .22 lever action rifle ever made, or ever likely to be made.
It's still made pretty much the same as it was in the 1890's, from forged and milled steel and American walnut.
Accuracy is so good it's often near Match rifle grade.