I'm with Xausa on this one, I think that a spacer will not be asthetically pleasing.
Further I think that to get a very good wood spacer is going to be very expensive and perhaps it would be more economical to restock, duplicating the origional.
I personaly do not have an problem with re-stocked "collectors" guns if the stock has been done in keeping. For the origional could have been broken in a fall etc at any time in its life.

Regards