That youtube vid sounds very like my bike!
It starts off idling nicely and no noticeable noise. building the rpm to around 2k the knocking starts, then 3k rattling. When I rode it last, there was a drop in power, but when I first set off it was ok???
Some time ago, I have spotted a "almost new" F800GS engine being sold on eBay. Somebody wanted it to power a Buggy.
I roughly remember it went for ca 2500,-
If there is no way how to repare yours, there might be be a possibility to get a spare engine cheaper than what those Bavarian thieves are asking for.
TBH if i had to replace an engine id probably buy a whole brand new bike, use the new engine in my bike then sell all the other new parts seperately. Youll probably find youll actually make a profit !
I know MCN did this a while ago (ok about 6 years ago) with a brand new ZX6R. Bike cost 6k... sum of parts once stripped and sold was 23k !