I hope the broken valve did not damage the piston....
Sorry, I can not offer the solution. 😕

 

"Severe knocking" could be a lot of things, and severity may be subjective.

Does it have enough oil? (Sight glass on the right side should show oil near the top of the inner circle when on the center stand after 5-10 minutes of being parked.)
Does it still run nearly normal except for the noise?
Does it exhibit a lack of power?
Does the left hand header get hot (and hotter or colder than the right?)

You can confirm almost exactly where the noise is originating from if the engine still runs by using a long screwdriver, placing the tip against various places of the cylinder and valve cover, then toching the handle to your ear like a stethescope.

 

Possibly this

https://www.r1200rforum.com/threads/cam-shaft-failure-bad-metal.57320/

 

Sometimes a cylinder not running will exhibit as noisy. Like @r0ckrat asked, what are the other symptoms? Is the cylinder actually firing?

It’s very easy to pop the valve cover off and check for any obvious mechanical abnormalities in the valve gear. Just be prepared for some oil to spill.

Unlikely to be ‘cheese’ cams - this was a very short duration issue well prior to your model.

I’d suggest no-one is going to buy the bike for a reasonable price with an undiagnosed issue such as that you describe, so you’ll need to somehow work it out. Do you have any mechanically adept biker mates? A dealer nearby?

 

Where are you?

 

Hi again, have tinkered about with a bike tech friend of mine and he suspects that the engine has a bottom end problem. So I have come to the decision to sell it as it is. So if anybody is interested let me know. This bike has been regularly serviced , oil and filter changes every 5k to 6k miles.

 

That’s a highly unusual failure, especially if properly serviced as you say (never heard of such).

You didn’t say how many kilometres the bike has done …

 

The bike has done 39k miles, and not hard miles.

 

I’d seek a second opinion

 

I would also get another opinion.
Have someone do more than listen to it.

 

Take the valve cover off and check the intermedia gear shaft that turns both cams.
Grab it and move it up and down and check for excessive play
That will give you a knocking noise(will be my first check).
The knocking noise will be more pronounced at idle.

 

Did You drain the oil
If not, drain it in a clean container and look if you see any shiny particles.
You can even go as far as cutting the oil filter for inspection.