At over 8 pounds, without engine, I’d go with a .91.

The 1.20 is about ¼” wider, longer, and taller than the .61 or .91 so it could present mounting issues depending on the design of the plane.

You might consider the MVVS .91, same size and weight as the OS.61. This link is to a seller in the UK:
http://www.justengines.unseen.org/mvvs.htm
They are made on the Czech Republic and, like many European engines, are designed for fuel with little or no nitro. If fuel with nitro is difficult to get in Africa this might be important to you.