Restoring a BeoMaster 1900 is a fantastic project! For authentic parts, your best bet is to keep an eye on online auctions and specialty audio forums where enthusiasts might be selling spare parts. Additionally, reaching out to Bang & Olufsen repair shops or specialists can provide valuable leads or even hard-to-find components.
As for maintenance, construct sure to handle the electronic internals with care — old capacitors and wiring can be fragile. Cleaning potentiometers and switches with a quality contact cleaner can help resolve scratchy volume controls and intermittent switch behavior.
One challenge you might face is aligning the tuner properly if it's off frequency. This requires some specialized tools and knowledge about RF alignment. However, the results are incredibly rewarding once you see (and hear) everything functioning smoothly again!
Good luck with your restoration! It's always amazing to bring these classic pieces back to life.