I have a warm morning (521) I bought this summer to put in my garage, but haven't burnt it yet. It has some buildup on the firebrick, though probably not enough to cause bridging. Some of the brick are in pretty bad shape too.
I think the best way to get the buildup off without removing the brick may be to use an portable angle grinder rather than a bench grinder. Borrow, rent, or buy one at Harbor Freight. I got one a year ago and couldn't live without it now.
I haven't found a cheap source for brick either. Have been thinking of trying to cast my own like was mentioned above. Good luck.
I had a few links to some castable refractory brick info below.http://www.visi.com/~darus/foundry2/refractory.htmlhttp://www.handymanusa.com/forum_five_msg/3160.htm