Anyone Repowered Athearn's old 0-4-2T ??

Athearn’s 0-4-2T “Little Monster” was my first HO loco back in 1962. It was known for its poor low-speed running, and I’d like to have a good-running example, so I’ll probably have to repower.

Has anyone successfully repowered/regeared this engine for smooth, slow running? Any tips on the best way to go about it?

Thanks,

D