Bachmann Thomas help

[Ty.the.railroad.guy]

Hey,
I’ve had a bachmann Thomas for a couple of years now and I’ve just recently started collecting the merchandise again, but I’ve run into a problem. I’ve noticed he runs very slow and I have to turn the speed pack almost halfway just to get him running. I did open him up a few times just to see what’s wrong and I did end up cleaning off most of the grease build up he had on his gears but it didn’t help. My track is fine since I clean it bi-weekly or monthly and I also recently got a bachmann James and Daisy last month and they both run great. Is there any part on the loco I could just replace from the parts store with? Or should I just get a new model?
Thanks