Merry X-mas Dave, and welcome to this forum,
My 1st generation Castile had the original ceramic fire pot that developed a crack in it as well. It's a testament to the early Quads that yours has served you well for as long as it has. The new cast replacements are pricey, but certainly worth it, IMO. It looks like you will also need the replacement linkage kit part #812-3120 and the loop igniter part #812-3811. Here's the Amazon site selling all three parts as an upgrade package for the same price or less than others were charging for just the fire pot.
http://www.amazon.com/gp/product/B00NOAHHIQ/?tag=trendjogja02-20
Quad reconfigured the air hole design and igniter placement to improve the forced draft through the fire pot and the igniter slit, which both reduces nuisance shut-downs and improves the burn efficiency, and is a lot easier to keep clean. I periodically spritz my cast fire pot with non-stick cooking spray and rarely have to scrape it, though burning good quality 100% softwood pellets helps with that too. I also try to pull the fire pot clean out rod after shut down while it is still hot, (I wait until the convection blower stops), before any fly ash carbon impurities can 'cold weld' in the bottom of the fire pot.
When you install your new burn pot make sure you get a new burn pot gasket if the fire pot doesn't come with one, and put some high temp 'anti-seize' on the bolts / nuts that secure the fire pot in, so you can get the pot out if necessary down the road. I had to use a dremel cutter disc to get my old ceramic fire pot out, as the heat had 'welded' the bolts on to the studs. But other than that the new pot install was easy.
Check the amount of clean out 'trap door' play you have in your new fire pot before you install it. Mine was good out of the box, but have heard they can sometimes be either too tight or too loose at the hinge assembly, which can either make the clean out plate too hard to open over time, or it lets too much air in if it's too loose. There should be no more than a dimes width - ideally just slightly less - at the side opposite of the hinge, which you can adjust via the nut / bolt / hinge assembly. Another new forum member who recently posted here had a 1/4" gap on the fire pot in his less than a year old Castile, among other problems. They don't make 'em like your retired 800 or my Castile models I guess !
Good luck, and post back how the new fire pot works in your vintage Quad. Cheers, DK