I am anything but a programmer, especially .dll's. I have no idea how difficult it would be to recompile the Hornet_FCS.dll for 64 bit, or if it's even necessary.
A suggestion, though, if I may be so bold: The Hornet_FCS.dll included in the v16.1 package is a relatively new one, updated by Orion. Try installing it into the correct location, instead of the older, "original" Hornet_FCS.dll from when FSX Acceleration first came out. As far as I know, the auto-installer will install the newer version into the correct location for you. If not, you can point the auto-installer to an empty "Temp" folder, let it install everything, then place the .dll manually.
Also, something I always double check on: The auto-installer will install the correct Microsoft C++ Redistributable 2015, if you let it, and you absolutely must. It does this after it's installed the rest of the plane. Sometimes, the installer for the Redistributable can take a while. It can
appear to be frozen, or locked up. It is NOT. Just let it run. it can take up to 15 minutes or longer. Just let it go until it's finished.
This is required to get the HUD and FCS to function at all, let alone correctly, as much as the .dll is.
Honestly, Orion is the "expert" at all things programming. I'm sure he will ring in eventually on all this. Just give him time

Hope this all helps a little...
Pat☺