Hi Steve,
Looks like the Fokker B.II used a 80hp Oberursel U.0 engine. I think this is the same 7 cylinder engine that comes in the Eduard Fokker E.II/III kit, which features a 7 cylinder engine for the Fok. E.I. You should be able to find the Weekender kit for less than the price of an aftermarket resin engine, and you will still be able to build the Fok. E.III or use the detail parts to upgrade some of the Special Hobby parts.
Dave