It's likely that CTL used a standard blade--not one just made for them.
I had a Grasshopper 52--it used an 18" blade and from my experience in a Toro shop, the Toro and Great Dane 52's used 18's as well. Knowing the correct center hole diameter you should be able to come up with something that will work.
If you're still concerned you could do some creative measuring to find the distance from one spindle center hole to the adjacent one--should be slightly more than 18"--1/4 to 1/2" would be my guess if the unit uses 18" blades.
Just a random thought.