I've been looking all over for a working LC 550 logic board (Part #661-1830). I've got three Color Classics — 2 working and 1 non — and I thought I could upgrade one of them with an LC 550 logic board to increase its performance without any serious modifications. (I've got an iMac G4 if I *really* need performance out of a Classic app.) Thing is, the 550-series Macs show up very rarely for sale on eBay/Craigslist/etc., and the best lead I've had was from a web page I found on Google that hadn't been updated since Clinton was president. (http://www.micromac.com/products/lb_lc550_upgrade.html — I called them and they said they'd look to see if they had a board and call me back, but I'm not exactly optimistic.) Even though I've seen a few 575 boards, I don't want to do the Mystic because I want to be able to use the stock CC back plate. I'm not even planning on doing the VGA sense lines mod — the analogue board on the non-working CC still works, so if I pop a 550 board in there it should just feel like a stock CC — but FASTER! 
Does anyone here have a spare 550 board? Maybe a spare Performa 550 or 560 or LC 550? I'll buy the whole machine if I have to; I just need to find one first! lol
Found one!