Is it possible to just replace the outdoor unit if that is the only problem and still use the existing indoor unit?
Technically there's no reason that we're aware of that would prevent just the replacement of the outside unit - but you'll really have to talk to a service technician about this because we can't speak for every single cassette air conditioner - and you'd likely need one that's an ideal match to the indoor unit.
Your best bet is to call the manufacturer or go to their website, and ask them to recommend a qualified repair person in your area - especially if it's still under warranty.
If the manufacturer can't help with that, just remember that whoever you get to service your air conditioner will need to be licensed to do so.
Hope this helps,
The BUILD team