I can speak from experience. Belt slip (or maybe it was valve stem breakage) at 70mph in top gear, totally destroyed one of the heads. Piston looked like it had been hit a million times with a sharp hammer. Rebuild cost was £££ x lots.
I have a complete engine for sale thats done 6000 with everything you need to run it if you need a new engine for an 899 Its good to go, i have heard of rebuilds costing 4000 region cheers ben
I dropped a valve on my 916 sp. it mashed the piston and head. I took the head to a motor engineers in Northalerton it was repaired like new. He got me some one piece forged valves from somewhere he uses ( He does a lot of car racing engine work) wouldn't tell me who or where it was. The bloke does valves for Italian bikes. I got another set for the vertical head. They were less than half the price of Ducati valves
Here's my original thread with photos of damage. http://ducatiforum.co.uk/threads/check-out-this-900ss-piston-damage.22369/
As has been pointed out, you would be best to by a sh head rather than new bits cost wise. I am sure its all fixable with replacement parts.