In case it helps, the Police Specification for the Clubman GT (SPO28) is documented in the Clubman Parts Catalogue PUB1052 (extract page below). This may be of use when you submit that the power plant was BMC factory fitted to a similar vehicle to yours.
The original pressed metal extractors are no longer available new and are hard to get (and pricey) so I wouldn't think there would be a problem using an equivalent aftermarket version.
All the other cautionary notes made by others above should be considered. Some people follow the "full disclosure" model whilst others use a more common-sense approach to things that they don't consider especially relevant in the circumstances. You need to do your own risk assessment.
Good luck.
Attachment:
SPO 28 v3.jpg