I have an OS 120 FS in my Venus II. No you do not need to do anything to the firewall. You could use the mount that comes with the plane but I choose to use a Dubro 686 only because this is the mount I have been using for years with good luck. It shows the 120 mounted at an angle in the manual but I choose to mount mine fully inverted. This meant doing some surgery to lower the fuel tank and adding some exhaust pieces. All in all I am very happy and glad I went the extra mile to fully invert my engine, they look like C#@* hanging out the side, I posted some pics of my install earlier in the thread but here they are again.

If you wish to add the exhaust manifold & flex pipe they are available at Towers and I can give you the part #'s if you wish. Did you give up on the .91 ????

P.S. If you choose to invert it you almost have to install some kind of external connection for the glow driver or keep reaching under the plane to connect / disconnect the glow driver. This is a pain in the A#@ and very dangerous. There are several different kinds and are cheap.