if you are talking about the ARF:

Tower trainer 40 ARF with 61 FX not a good idea.

i'm not sure the tower trainer 40 ARF engine mount would accept the 61 FX!!!!
and the fire wall is designed for that mount only!!!

why not to take the 60 size trainer with that motor or the hobbico hobby star 60?

tower trainer is a very good airplane! (had two of them) one still flying.

more significant change would be replace the nose gear at least, no all the landing gear, the main and the nose and to recover it!

if you build it from kit then it will accept other engine mount, then you can put there the 61 FX.

besides that Tower trainer 40 fly's very well with 46 AX, mine flew really well with 46 LA.

Alex