Very nice,,excellent find!

Study the disassembly of the magazine cap & lock assembly before taking it apart to replace the forend wood.You have to remove the band, the lock 'pin' assembly & screws ect.
The forend wood still attached to the slide comes off with the mag tube still inside of it.
Then you need to unscrew the end-cap on the forend with a spanner wrench, remove the endcap and slide the old wood off.
The spanner can be bought from places like Brownells,,or you can make the spanner from a suitable piece of thin walled pipe as I do for the necessary jobs. The 'pipe' style spanners lock in the bench vise and you turn the forend assembly to unscrew the cap.
The spanners are off-set teeth and though they work, I always found them more liable to skip out of the slots on the end cap.
Either works though.

Some wood will interchange w/o any fitting. Sometimes you have to trim it just a bit to be able to re-install the endcap with the spanner to tighten it and get the bbl cut-out on the cap lined up OK.

There's 12ga forend size,,,and then there's a 16/20 size.

If the wood won't slide easily onto the sheetmetal pump slide assembly, you have to enlarge the inside of the forend just a bit. Since it's simply round, a coarse sandpaper on a mandrel using a drill to spin it cuts quickly and cleanly to do the job. Don't over do it and watch that you don't funnel out the ends oversize.

Sometimes it's just plain easier with a small crack or missing piece such as on yours to repair the original if much fitting is demanded by the replacement..

Just some thoughts.