Agreed it likely is a 66 model.-----Those were painted green starting in 1967 model year.-----Do a complete evaluation before spending any coins on it.
Take the motor apart.-----Do the honing.-----Do the inspection / measuring.-----Determine the cause of the failure.-----Install the latest water pump kit.-----Your " guy " will know all this.-----If he hesitates then find another guy.
I have worked on these simple magnetos since about 1968.-----Best for you to visit ----leroysramblings-----For the crash course on this simple motor and magneto.
The factory condenser are very reliable.------What other trouble shooting was done ?----Did you install new coils , yes or no ?-----New wires threaded into the coils , yes or no ?
Step #1 is a compression test.-----Post your numbers.-----Next is a spark test.-----Does spark jump a gap of 3/8" on all leads ?-----I chuckle when I read ----" rebuilt the carburetors , motor still runs the same ".
Use a proper spark testing device.---Easy to make with wire and piece of wood , stick , tape at home.----Spark on a good system should jump a gap of 3/8" or more.