While you clearly need better track than the plastic, one suggestion would be to buy a modern version of the Bachmann 4-6-0. There are thousands around and they can be acquired for $50-$100. They have had continual upgrades, so the 'modern' ones are Anniversary or "Gen-5", characterized by a hump under the gear on the rear axle.what is a relatively cheap steam G-Scale Locomotive (steam locomotive), with a large amount of pulling power, that can easily be converted to dead rail operation?
Then you need to disconnect the track pickups, which is very easy on this engine - the front truck has wires that you can snip, and the bottom plate has wipers that rest on the bearings and they can be unplugged. While you have the bottom plate off, connect a pair of wires to the track pickup wires leading to the motor and run them out the back.
The tender can take as many batteries as you want to install. Then, for a control system, you can make it manual by using a PWM motor speed control (a few $ on eBay or Aliexpress) or a simple radio controlled ESC. A cheap car system works but doesn't reverse too well.