Were you trying to stick to Part 103? If so, my numbers say 22% scale for a slightly higher wing loading than a Lazair with a wing area of 155 sq ft and a gross weight of 500 lb. That would still be a 40’ wingspan so not small. If it were me I’d go with six giant scale electric RC model engines and a removable battery pack as those Saitos will start to get very expensive.
A better candidate for the silly scale multi engine ultralight would be a Miles M.57 Aerovan, which first flew in January 1945 but didn’t enter production until 1946. At the same weight and wing area it would be about 63% scale with a span of just 31’ 6” so much easier to handle and store. Completely impractical but good fun…!