Beurling flew nine different serialed Spitfires (both VB and VC) while stationed at Malta. A few of these can be found in aftermarket decal sets.
BR323 was an option in the Special Hobby kit SH48051. Only other choice I can find is buying separate decal sheets, one for the white fuselage code, and the other a generic sheet of 8 inch black serial codes.
Hannants in the UK should provide some choices in their RAF decal sheets, but if you want to try spray masks, Maketar has some options for code letters in a variety of heights.
regards,
Jack