The Revell 1:72 N28 is a very nice kit for its age and has been in and out of the catalogue lots of times with various decal options. From what you say you have the the one which includes Campbell's machine. In the initial release this was incorrectly depicted as light grey (possibley after a preserved example which was around in the sixties) and thus had black numbers. I don't/can't do 1/72 anymore, so I don't know for certain, but there ought to be an after market set of USAS style numbers surely; perhaps from Blue Rider. You may also want to post your question down in the models section, one floor below, were there are some still able to see in smaller scales

. Matt Bittner did a nice build for
Internet Modeler and is a regular down there. Otherwise it's a fairly simple style of numbering and would be easy enough to cut from white decal stock or some spare rudder striping.
hth