Michael,
I can scan the CTC-4 schematic from my RCA color service guide. I do not have Riders for RCA color past CT-100. You will have to gimme a few days to do this. Please contact me with an email address where I should send it.
You can swap any of the glass 21" CRT's for the metal 21AXP22. These include 21CYP22, 21FBP22, 21FJP22. The CYP needs a 56K, 2W resistor to the second ultor connection nearest near the neck from the other ultor connector that gets the full HV.
Fixin' up a CTC-4 eh? Good stuff! We expect pictures when completed.
Note all the crt's except the 21FJP22 need to hide behind a separate safety glass.
Rob