Had a similar deal with the Hobbistar. Its now got iron on film, so they give you a decal with the reg number and leave a gap in the trim on the side of the plane.

The numbers are sloping (itallic?) and slope in opposite directions of each side. I just went to the craft shop and bought 2" vinyl letters. The letters are straight but they do fit...

BTW those letters are a pain to put on and line up....