Really a lot of potential there, nice bicycle. Measure the chain after you clean it and replace with a new one if its beyond tolerance. A good chain will make a difference in the shifting and in the bicycles performance. Cables might be another inexpensive part to go ahead and replace with some new items. last thing I can think of is to clean and repack all of the bearings with a good waterproof grease of your choice.
Should be a nice ride when you get it cleaned and serviced. Hope that you enjoy it.
Bill
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Semper Fi, USMC, 1975-1977
I Can Do All Things Through Him, Who Gives Me Strength. Philippians 4:13