Correct, the spline is different but easy to change, I have an extra Sirocco wheel if someone would like it, reasonable, needs to be re-covered
I wouldn't call it conversion.
The scirocco wheel is the same kind of petri wheel used in the csl, except of the used material. The scirocco one has a foam with a leather structure and the csl-petri has real leather. (for my knowledge - if I'm wrong here, pls correct me)
The metal part and adaption to a hub is exactly the same. So find a original Petri hub fitting to the BMW steering column or adapt it yourself to another hub.
I did it with an original Momo - hub and it wasn't that difficult. The bolt pattern of the Petri is a little bigger than the one of the Momo wheel, but there's still enough material to drill this specific bolt pattern.
In addition I put some leather onto the scirocco wheel. This was a reasonable way to get a CSL-looking wheel for a relatively low price ....
cheers
Ingo
and by the way EGOBGO - aka Big Ego, is really expensive.