One option might be to make inkjet decals if someone responding has a set but wants to use them themselves and can scan them for you. However, making light colored markings is much harder than dark colored markings, and most P-39s I am familiar with had light markings.
It is not impossible to make light colored decals, just more of a challenge. You must print them on white decal paper. To make it possible to cut out small markings you must use a background color corresponding to the color they will be used over, and color matching and calibrating your inkjet printer for accurate color reproduction is a challenge.