You might try Nova darkroom as well if Northernphoto can't help. Nova used to keep "bits" for enlargers. I got a spare under-the-lens red filter there some two years ago. Best of luck
Just a thought on the 6x6 insert. If the worst comes to the worst and you cannot find an insert separately, then a machine shop should easily be able to produce one if you can obtain the measurements of an insert or better still borrow an insert and have a copy made.
I only have one insert which I use frequently, otherwise I'd send you mine to enable a copy to be made
I guess the colour head off my spare Durst would fit, it's an M601 with I think a CLS66, there must be a lot of them around. Unfortunately I'm keeping it, pending a possible darkroom facility where its warm