If you can afford to spend the money, Evaporust or a similar brand would be the way to go to derust that bike.
All plating is not the same. I have used Lime juice with good results, mainly because I am frugal.[cheap is a better word], but since this is dad's bike, you may want to go the "safer" route on your rust removal. Plating is a not so funny thing, once it's gone...it's gone. If you read the Restoration Tips posts you will here lots of good things about Oxalic Acid. It's cheap and does wonders on rust removal , but DO NOT USE IT on plated parts, Never. It can remove too much and is inconsistent depending on temperature and mixture ratio.
I had a hunch your dad was a Korean War Vet. These guys came back and had lots of catching up to do and did that sort of thing , especially with cars and motorcycles. To a lesser degree with bicycles, depending on their budget. Thanks for sharing the story, and the bike. God bless your pop and thanks for his service to our country.
Keep us posted about the bike.