Hi
I think the B series diesel was phased out before I was born so i dont know much about it to be honest, and i think all the sherpa vans will have been scrapped because of rust by now.
Though, according to wikipedia the diesel is still used in canal boats so the best places to look would probably be marina/ sherpa forums or search forums for canal boaters maybe.
hope this is of some help anyway
just had a look on ebay, found these
http://cgi.ebay.co.uk/BMC-1-8-MARINE...item2a1068a215
http://cgi.ebay.co.uk/BMC-Engine-150...item3cb7c12b38
so the engines are out there still, at least! have you considered the replacement foe the b series diesel, the perkins prima (2 litre i think)? may be suitable fit.