Proton used to use engines it built under license from other manufacturers (notably Mitsubishi). In those days, there was a decent chance that a Proton rocker cover would have been interchangeable from another make or model that also used the same or similar Mitsubishi-designed engine.

Eventually, though, Proton progressed to the point where it was building and fitting engines of its own design, and the one in your Preve is that first of those. As such, you probably wouldn’t be able to switch it for another make and model’s cover, although you might find that other Proton models from the same era as your car (and which used the same engine) will, in fact, have rocker covers that can be mixed and matched. But be very careful, as even a different location for the EGR valve or breather tube might be the difference that renders the two covers non-interchangeable.

Just because Proton isn’t selling cars in Australia any longer (the brand disappeared in 2017) doesn’t mean replacement parts can’t be obtained. Even if you can’t find the rocker cover gasket you need in a conventional parts store, there are lots of online sellers (both here and overseas) who can supply what you need.

The engine in your car was hardly an orphan, either, and was found in many other Proton models including the Waja, Gen-2, Persona, Saga, Satria and others, so don’t rule out a search for those models too, as the rocker cover gasket should be interchangeable between them.

The Preve was not a huge success in this country, which is probably why many parts outlets don't carry spares for it in Australia. But if you jump online, you'll soon find plenty of overseas suppliers selling what you're looking for.

Alternatively, you can ask a transmission specialist to investigate supply of the cooler. Perhaps that same shop can match the cooler with a part number from another car that will fit and do the same job. As a last resort, there's every chance a completely different cooler could be adapted to fit your car. This could take some fiddling and workshop time, but if it keeps the car on the road, it's probably justified.