The ca’s never had a floor mount gearlever, however I have seen some conversions that basically stick the gearlever through a hole in the floor. Nothing is available off the shelf, you have to make and mend.
The top drive is exactly the same in the 3 or 4 speed boxes anyway, unless you are driving round fully laden up 1 in 3 hills, I find the 3 speed fine
I think the Zenith 36iv will work, as long as the jets are correct – Just see how it runs, Ca’s are not exactly high technology.
Better conversion is to go for a weber – see earlier posts in this forum