Thanks for your email; and for your patience whilst I passed on the enquiry to both the Classic MX and MX/SX Commission for comment. Both have confirmed that your brake system is suitable.
For the MX Commission it simply needs to be a brake that “works” i.e. slows down that rear wheel to assist with stopping.
The Classic MX Commission do not require you to have brakes at all (as per the GCR’s) however should your machine be deemed “unsafe” on the track, the Clerk of the Course has the right to remove it. You would not be deemed unsafe on this machine as it has been presented.
Hopefully this answers your questions, if you need anything further please let me know. Best of luck with racing!