You mean you can't do transposition on-the-fly, in your head?
Seriously, I know some folks who can do a pretty job at that playing piano. I have no idea how their brains work. Since I play sax and need everything transposed to play I re-type all my music into MUP which does a very good job of transposing.
Sorry it took so long to respond but yes I can transpose on the fly IF the song is not to complicated. I could never do it on a song like Roll the Dice.
Fortunately I can read music and I can jam and improvise on the fly so if I am having problems transposing a lead line I can just jam or improvise. In other words fake it!
A true story. Many years ago I was playing in a wedding band. We were playing the song Fascination with a Tenor doing the lead. The key was Eb. The lead player looked at me and said take it. Well I really messed up the first run (it-was-fas-ci) so I tremolo picked the long notes (na-tion-I-know) (for those that know the song know what I am talking about). People started to laugh so I purposely blew all the runs and tremolo picked the long notes. Everyone was laughing, including all in the band. When we got done everyone was applauding. On break many said "you are a really good guitarist. I liked the way you purposely messed up that song but came back right on. You gotta be good to do that". If they only knew the truth!