If the insulation of the original wire is not exposed or brittle, then yes, resoldering can be a good thing. Old solder does sometimes break away or becomes lose and your electrical connection is impaired. And since you have it apart, it would be a great time to give it a really good cleaning and oiling. Check the gears for any hair, fur, or lose material and remove with pointy tweezers. Lubricate the gears with a light grease that you can purchase at your local hobby retailer, or online through eBay. Lionel sells a lubrication kit with grease, oil, and track clean. That might be your best bet to purchase. When you’re all done, your locomotive should operate like brand new when it first came off the assembly line. Please advise how your project turns out. It will be a fun thing to do, and self gratifying. Happy Thanksgiving!!