Tinus Tuning makes adaptor plates to fit the T9 to the B18/B20.
https://www.tinustuning.nl/1-onderde...ndrijving.html
Fire off an email to him and he may be able to provide all the details associated with the conversion and maybe he can put together a package of the required stuff.
By the time you source everything it is not going to be cheap. On this side of the Atlantic, the only prices I could find for the T9 suggest that you are going to be looking from a minimum or around $700 US$ for a salvage box to around $2000 - $3000 US$ for a reconditioned box. I now remember thinking about this for my 142 about 10+ years ago and deciding that my M41 was probably just fine. The cost is also probably the reason people go with the T5 on this side of the big water.
I think Tinus Tuning also gives you a T5 option behind the B20. The other party offering these adapters is John Parker at Vintage Performance Developments (VPD). VPD is a dodgy supplier without par (take the money and run) so I would stick with Tinus Tuning because he appears to actually deliver parts.