Tough call on that one.
Book value?? doesn't even list anymore - a mid 70's model is worth about 160-175 bucks.
Some parts of the ignition system on this model, the rotor in particular, is no longer made and very, very difficult to find (and sold for over 300 bucks when it was available). If that goes you have a couple hundred pounds of scrap aluminum on your hands.
So there is kinda two values for this motor. 1) to be used as an everyday motor, maybe 100 bucks if it's running right now. 2) for someone with a classic boat that want's the "original look" and they have maybe one or two others that they use to cobble together the one they keep on their boat - maybe 200 bucks.
There isn't much out there these days that require a short shaft in this power range - 100 brake horsepower (1969) equal about a modern 90 shaft horsepower motor.
Since you are looking at dealing with a "friend". Unless you have actually been using the motor, why not toss it his way for a case of beer or something. You wouldn't want bad feelings if it goes "flooey" once he starts using it and then learns that it could take thousands (and tons of looking) to try and find parts.
Like I said, unless this was going on a "classic" simply for the look, this is the type of motor I would advise to stay very far away from unless you got it for nothing...