Fairly straightforward to get a piston engine to run backwards. Typically just involves left and right hand rotation camshafts and distributors/magnetos.
Of course, that assumes the engine has been designed to run that way from the outset, which many industrial style engines are. Some things that need to be considered when designing such an engine are oil pump drive and cam shaft drive. Oil pump is easily fixed by direct crank driven pumps and the cam drive can easily be overcome by a chain with no tensioner or gear drive. But there are also many one off reverse rotation conversions out there that work well, it just takes more work to make it happen.
The older two stroke Detroit diesels can be easily changed to opposite rotation and was reasonably common to order LH and RH rotation engines in marine applications.