Forget about getting a drawing with part numbers; you won't get one.
But the usual diagrams can be found in the usual repair manuals.
Since it might be difficult to buy a new one for the T3, you'll probably have to look for it on the used market.
Oh, there's something new after all!
http://www.amazon.de/exec/obidos/ASIN/3768804119/qid=1110403179/sr=8-1/ref=sr_8_xs_ap_i1_xgl/028-7426972-6102129
früher, there were
http://www.volksteile.de/, which also had markings, but for a reason I don't know, that page no longer exists.
If you're looking for information that isn't specific to buses, information about the JR engine in the Volkswagen Golf 2 might also be helpful, as the engines are very similar.
You can also ask questions here if you have any specific inquiries.
m;