Ditto what greenjeans and Steve-o says. It will be less valuable than a factory stamped engine but more valuable than one that was restamped without the Honda font as was often done back in the day. Probably highly unlikely to ever know the original engine number unless the state(s) it was titled in recorded the engine VIN as well. I have a similar issue on one of mine except the engine number was restamped on the right side on the bottom of the cylinders. So at least I think I know what the original engine number was.
Tango911 is getting a pretty big reception over on the Sandcast Only forum as 3-digit barn finds are extremely rare, especially along with most of the original parts.