OK, the rumor about the existence of a retro kit for 6600 is false.
No such animal.

My employer has a company policy that there is to be no business conducted with ebay or other online market places.
Chicago Engines is one of the firms we contacted and we were told that they no longer have any long block nor complete 1.6 engines.
Short block and separate head are the only way we have been finding the engines available.
It is easy to be misled by visiting a website and see that a lot of stuff is "offered" only to get the real story when contact by phone or e-mail is made.

Right from the start of this I have had the gut feeling that this specific engine (the one in our sweeper) may have suffered a warped head during the events leading up to the very first "head gasket" failure in 2009.
Though this is a Tier II engine with overheat limp mode provision, that does not guarantee the engine cannot be subjected to damage causing overheat conditions. If the machine is placed out of service and repaired properly AT THE FIRST INDICATION of trouble THE VERY FIRST TIME it goes to limp mode, there is a chance that no major damage will be incurred.
But, knowing the "denial mentality" of the average dock worker or dock supervisor regarding vehicle maintenance, it is very likely that back in 2009, the engine could have been subjected to multiple overheat events before the sweeper was presented to the shop with an issue.
Realize that each overheat event can expel coolant from the radiator into the recovery bottle, overflowing the recovery bottle and spilling some coolant out.
Repeat that several times a day and pretty soon you have so little coolant in the engine that the ECM temp sensor bulb is not submerged in coolant (which means the head is dry) and the temp signal of overheat will be late in reaching the ECM.
I have a scene in my mind of the operators simply shutting off the engine and letting it cool down a while and then going right back to sweeping some time later and never reporting the problem. They detest using a push broom, and there's really no way a push broom could replace a sweeper machine realistically.

The repair work that was done in 2009 was done by a different tech (since retired) and a different regime of supervisors.
In 2009, the head was not sent out to a machine shop for testing/checking. In 2010, the head gasket blew again and same tech replaced it again without having head checked.
In 2012, I removed the head to extract a broken spark plug (broke while trying to remove plug) and I detected the early signs of head gasket failure on cyl #4 between the back edge of cylinder and the coolant riser passage at the rear of the block/head. That was the cylinder having the broken spark plug too. I was able to extract the remains of the plug without resorting to an insert and management would not OK to send the head out for testing/checking.
The head gasket failure this time is precisely at the exact same place I saw when I had the head off in 2012.

We now have a different group of managers including the GenMan, so things are being handled a little differently this time.
The hours on this engine are into the 16,000s and considering the multiple head gasket failures, it was suggested that a new engine would probably be the shortest path to return the sweeper to service.
Having met with some frustration trying to find a drop in engine, we have now decided to repair this one, but not following the previous methods.
Monday, the head is going to a qualified automotive machine shop for thorough testing and if it passes muster, it will get the valves refaced and new stem seals.
I am going to pull the pan and look at the rods and mains.
Thanks all and I will update on how it goes. A sweeper has been rented to cover the down period (I told you a broom does not fit their hands).