Who said it is not registered with CASA? it is not an annual fee it needs to be cancelled. For that matter insurance too.
It is very possible it is both (not road registered I assume).
I hope the Authorities take it as a fun thing as possibly intended, but I doubt that. That said I hope it is not CASA register, penalties much higher (taxi approval/running engine x distance from building/ no STC or workpack entry for fuel system mod).