Hell yeah, go for it! Dont live a life wondering "what if I only had the guts?".
Worst case, she says no thanks, and you move on and dont have a question hanging over ya. Best case, she shares the feelings and you have a relationship starting with a girl that you like who also likes airplanes. As far as her fiance goes, if she is interested in you now, she obviously didnt care enough about him and its better he learned that early.
Waiting till after your checkride would probably have been a good idea, but you have made the move, now have the nads to see it through.
good luck,
dago