I have a 1948 chev woodie, many of woodies have little spots here an there that could be fixed. (After all its 75 years old!) I'd say leave it alone, that piece that needs to be changed is quite significant, it runs from that D column to the C column. To make the change would require the roof material to be removed to complete the work. Someone might be able to change a portion of the roof beam rather than whole beam, but that too would be quite expensive. I believe it is better to save to original wood than start changing out pieces.
Just my 2 cents, use some filler and re-varnish it.
