That's a pity. So no possibility of retriving of the letter. However you may try to paint that either on the fuselage or on a clear decal sheet. For both ways you will need to make a mask of the masking tape. But of course, thats' the last resort.
What I've done is apply a letter J decal in the correct scale ( unfortunately I only had one in grey) then I will mask it and paint it black. Voila!
Thanks for all your help.