First try cleaning and checking the sliders.
Lay a ruler across the two driver treads, there MUST be a gap between the slider and the ruler when the slider is pressed into the loco.
What usually happens is, dirt gets in the slider spring area. That holds the loco ever slightly off the rails, (not really enough for you to even see it, but all the weight of the loco ends up being held up by the slider instead of the wheels. The Forney usually has plenty of pulling power, even if the rubber tires are missing. If the tires are still there, replacing them won't help much. It might help a little, but not so much that you would notice, and any traction you gain by replacing traction tires that have not broken will quickly disappear as soon as they get a little dirty.
Clean the sliders out. You can sometimes just force a little track cleaning fluid into them and blow them out with compressed air. If they don't easily and freely compress when you press on them lightly, take off the bottom plate, remove the sliders and clean the area with a small paint brush, smoke fluid and compressed air. If the sliders are bent, straighten them or replace them.