Frank makes a tool that makes changing the o-rings easier (http://www.franksoutboardtools.com) I have done them with the dental tools but this thing helps.
I would make sure you check the coils if original probably cracked. Clean and adjust the points, clean out and rebuild the carb. Drop the lower unit and change the impeller they get set or bent and stay that way so do not pump well, can also get brittle and break up clogging the water lines, either way can over heat the motor in a hurry. Would probably change the oil in the lower unit and put new seals on the screws. From what I have seen with my 68 18hp they are real good running motors.
I would make sure you check the coils if original probably cracked. Clean and adjust the points, clean out and rebuild the carb. Drop the lower unit and change the impeller they get set or bent and stay that way so do not pump well, can also get brittle and break up clogging the water lines, either way can over heat the motor in a hurry. Would probably change the oil in the lower unit and put new seals on the screws. From what I have seen with my 68 18hp they are real good running motors.