The gearing isn't awful, it's probably geared like a scale model. Which is to say, not fast like so many toy trains being sold today. Before you choose a replacement motor you should figure out the gear ratio. It's a little hard to do without a flywheel. But if you make a little mark on the drive shaft and pay close attention, you can turn the motor shaft by hand and count the number of turns required for one revolution of the driving wheels.
The stock Mabuchi RS-550 is rated for 12000 RPM. The big 9000-series Pittmans are about a 7000 RPM motor. So your top speed will be much reduced. Meanwhile an 8x24-series Pittman is a 10000 RPM motor which is closer to your original, that's why I recommended it.
IMO the most important upgrade is a flywheel. Without it, the loco will stop on the proverbial dime. If you're going fast enough, it can cut a bur into the drive gear, or bend a rod. It's hard on those $%^#*&% rubber tires, too. Coasting, at least a little, is a good thing!