Andy ... its not a stupid question, you are just new. What do you mean that you have no idea how to mount it? What is the problem? Is it that the engine mount will not fit? The F40 was made for 40 sized engines, which include 70 sized 4C engines too ... the 91FS is larger and I believe the holes on the firewall will not be wide enough, you may need to remove the blind nuts behing the holes, plug up the holes and re install the blind nuts to fit the mount. Perhaps you need an experienced modeller to assist?