A 20 inch span Fokker is not going to have much wing area and with the larger equipment the loading would be high. I'll bet it high speed taxi'd all over but just never had a chance to run fast enough to fly. And yes the 7x3 may have reduced the revs and thus speed limited it so it did not want to take off. But if it had the regular sized gear in it I'm sure it was a bit heavy so perhaps for the sake of the radio gear the non takeoff was all for the best.

If I was doing an 049 Tripe I'd enlarge it until there was about 230 to 250 sq inches of area and build it with sticks with lightweight covering. Power would probably come from a 7x4 or 7x3 on the engine set up with a couple of extra head gaskets to reduce the compression ratio in order to make it more big prop friendly. The weight in this size should not go much over about 18 to 19 oz. At that size and weight it would fly fairly well and scale like. In otherwords you would need to dive slightly to do a loop but consecutive rolls would be easily done.