Here’s an easy solution for the engine that’s sparking. Wipe down the rollers with rubbing alcohol until they shine. Then, flip the loco over, and connect a wire from your transformer to the roller (use an alligator clip or loop the wire around it somehow), then connect another wire from the transformer to the drawbar or one of the trucks on the loco. The idea is to get the motor to run without touching the wheels. Turn on the power. You may have to experiment a little to find a place for the wires that will get the motor running.
Got it running? Good. Take a cotton swab, dip it in rubbing alcohol, and touch it to one of the wheels. Watch it turn dark from all the crud it cleans off. Get a new swab, and repeat until the swabs stay clean. Do this for each wheel. Once you’ve cleaned the wheels in this manner, that will almost completely eliminate the sparks, and will make the locomotive run smoother, since it’s getting better power.
The loco may need a lube job too, but most old trains definitely need the driver wheels cleaned. I’ve taken some that didn’t run, and cleaning the wheels was literally all it took to get them running again.