As Andres says, the one in the picture is a 2 valve engine. All the 4 valve engines have a black plastic cover which covers the rear 1/3rd of the valve cover.
After 2010 the 4 valve engines had harder followers fitted, but they were still prone to wear. It wasn't until the back end of 2012 that the roller followers were fitted to all engines.
The conversion to rollers wasn't a recall on early bikes, they would only do it under warranty if, there was a problem and the bike had been fully serviced at a Guzzi dealership. They will still supply the parts if the bike is fully serviced but the 3-4 hours labour has to be paid by the bike owner.
There are I believe 3 different kits for conversion depending on engine number, the average cost to pay a dealership to supply the kit and do the work is between £1200 and £1800.
Some non-rollerised 4 valve engines have been fine upto 40,000 mls some have only managed a few thousand before needing the work done. There are very few that have gone beyond 40,000.
In my mind a well sorted 1200 4 valve is a nicer engine than the 2 valve, smoother and a few more horses. Having said that, I prefer the look of the 1100 and early 1200 2 valve engine.
It's very easy to tell if a 1200 has been converted, just lift one of the rocker covers and it's plain to see if it's been 'Rollerised'. There are still a lot of early 4 valve 1200's with the flat tappets out there.
I've looked at 3 very nice bikes recently and none of them have been done.