Larger transformers by reputable firms - Lionel, American Flyer, MRC might be worth repairing. Only the larger Marx transformers over 100 watts ,and only those that have built-in circuit breakers, warrant repair. Buy from a dealer who knows his trains. He can make sure you get a transformer compatible with your particular train set. Track wires can be replaced with 18 gauge wire.
Most folks prefer stranded wire.
It can be bought at most electronics and hardware stores. The cost is minimal. As a rule, it would be a good idea to replace all wires, worn or not, at least every ten years. Look for corrosion, bends, breaks and missing clips or pins. Take a look at the lockon or terminal, to see if it is in good shape.