How'd I miss that!? I saw this repair(the one on your rifle) done on a L39 recently, it was unlike yours double dovetailed(ie. front and back, because unlike yours this damage must have ended before the back of the forend ) and pegged with fine dowels. It was beautifully done. I suppose with this one you only dovetail the front, cut the crack along the grain so that it's parallel with the top of the forend, fit an oversized patch in similar timber with the correct grain orientation, glue and clamp, allow to dry, rough shape, peg, make off.
The patch would only be as big as essential, but would be similar to this, but on one side of the other end.