Hello!
Seems like BCG is really active globally, since many people are asking about various steps of the process with them.
The online quant test are indeed very similar among them and hence, better to prep.
I would strongly recomment you practice it with the Integrated Reasoning part of the GMAT exam.
There are free exams in the internet that you can use for practice (the one of LBS MBA page, Verits prep, as well as some free trials for courses such as the one of The Economist (https://gmat.economist.com/)
The old McKinsey PST, Bain SOVA test, etc. are all quite similar so leverage resources across all of them.
https://www.myconsultingoffer.org/case-study-interview-prep/bcg-online/
https://www.psychometricinstitute.co.uk/Free-Aptitude-Tests.asp
Hope it helps!
Cheers,
Clara