Hiya Cod,
Having had my share of experience with the ignition housing (sometimes referred to as the ignition barrel on this side of the pond) and its components, I'll offer up my two cents worth............
There are a variety of considerations to mull over when replacing this component.
Quality. Over here in the States you would be wasting your time trying to replace any of the ignition components (housing, keyway, steering lock) with aftermarket parts. Why? Very poor quality in the aftermarket parts versus some of the highest quality OEM parts from VW. A simple cleaning and lubrication of the keyway and it is good to go. This rule of thumb does NOT hold true for the electrical switch however. The electrical switch is the weak link in this assembly and it alone is often the reason for problems.
Key Problems. Going with an aftermarket keyway will require a different key. That will mean you will no longer have a one key fits all locks on your Bus.
Fitting. As has been pointed out by Araon there are a variety of housings and components (actually more than three - VW was very secretive about yearly changes in this assembly for owner security) and any generic aftermarket part may need some customization in order to fit/work properly. Have you ever noticed how many POs have gone to a push button start system? Often because they either could not find an adequate replacement or could not get that replacement to work.
I'd suggest that you try to find the exact same ignition housing that you now have at a boneyard. Excellent quality and it will be an exact fit and engineered to your needs. Further, you can transplant your present keyway and drive off happily into the sunset.
