I love the old CZ's. My son-in-law just sold his 1973 CZ 400 motocross bike. It had the distinctive "coffin" tank. The manufacturing and design used on a 1973 CZ is pretty crude by today's standard. It was like a Schwinn Stingray with a big motor bolted in. My son-in-law does have the frame for a 1973 CZ Endduro. He planned to transfer parts from his MX bike to that and register it, but life got in the way.
There is a Czech fellow that owns a CZ parts shop in Whittier, Ca. I forgot the name. He claims that he imports and sells virtually EVERY CZ part in the USA. He showed us tons of orders from other resellers that order from him to prove it.
So, if you go CZ, This fellow in Whittier will likely be your connection.
I included a photo of the bike he sold while it was in my backyard.
Mike