Mark, the early Chinese-built Briggs engines have an unfavourable reputation for build quality. I doubt that is the cause of your lean mixture problem, but it does suggest you should not regard that engine as something marvellously valuable. In other words, this is a great opportunity to work on that carburetor and practise to increase your skills and experience. If the engine has lived this long, it may be a rare instance of an early one that by sheer luck, they built properly (or it could have even been an example built by experts as a training exercise, who knows?) That in itself may make it a future classic, but not for 20 years or so. Meanwhile I suggest you learn from it without feeling too badly if you make mistakes.
