Featherweight rifles in the condition you’ve described aren’t rare, so you don’t have a highly collectable item.   The addition of the muzzle brake kills the value of the barreled action.  The action alone would sell for around $550-$600.  To restore the rifle, you’d need to replace the barrel and the stock.  Barrel would cost between $100 to $200 for a used item.  A good original stock will run between $400-$600.  Gunsmithing would likely run about $200, on the cheap.  So, to restore, you’re looking at about $1000 out of pocket.  Even then, it might sell for around $1500 in today’s market.  I’d either keep it as a shooter or sell it with the expectation of getting about $700-$800 for it.  Just my opinion.

I might have been a bit premature with my first post.  I improperly assumed the aftermarket stock was a readily available add-on, like a Fajen.  If so, that would bring down the value of the gun compared to an original rifle, and I’d still stick with my $700-$800 estimate.  However, if the stock is truly a custom job with Grade XX wood and made by renowned stock maker, the value of the gun could go much higher.  I’ve seen several with beautiful after market stocks sell for $2000+, but you have to reach a wide market and find that particular buyer who really wants your particular rifle.