Would to know were to get replacement for ahm/rivarossia engines.
AHM has been out of the marketplace for 20 years or more and Rivarossi has been bankrupt and was then purchased by Hornby of England. Present day Rivarossi products are made in China.
About your only hope of finding repair parts is to frequent flea markets and train shows and try to find models that can be cannibalized for parts, but most of the AHM locomotives I’ve seen are not worth the time, effort, and expense of repair.
You’d be much better off purchasing new models with vastly improved detail and mechanisms instead of trying to repair something that is, technically, an antique of little value.