Yes, you can go on with your building... Just mount the fire wall and motor mount as per the instructions. When you put the blind nuts in place, glue them in. Put some petroleum jelly (Vaseline) in the blind nut to protect the threads from the glue. When you run the bolt in, after the glue is dry, the petroleum jelly will be pushed out. When you get your .61 FX mount it to the motor mount and check the balance of the plane. Just make sure your motor mount is for a .61 engine. You can tell some times by looking at the back of the motor mount. You will see something like 40-60 or 40-70 or 60-70 or 60-90. You get the point. If your mount is for a 40-60 size, you may want to take it to your LHS and ask to barrow one of the .61FX motors in the case to see if your mount is wide enough for the FX. The .61 FX crank case is just a hair wider than the other os .61 engines and it may be a tight fit. Most hobby shops will be more than happy to let you test fit your motor mount to a display motor.

I hope I was of help!

Good Luck, Happy building!