I have that set of plans. The plane is mostly stick-built so it is very light for its size. I have a Unionville Beaver with a 72" span on floats that has the same type of construction. It weighs only 6 3/4 lbs with the floats and is overpowered with a Saito 56. A 91 would be overkill. A 46 two-stroke or a 52 to 62 four-stroke would be plenty. There are photos of it in this forum under the thread "seaplane photos" page 10.

Cheers!

Jollidude