I agree with Steve.
It is not too late Andrew. The price of 20B's has gone way up recently.It will go higher as time goes on as Mazda is no longer selling 20 B
parts at reasonable cost. A 20B FW installation is 100 pounds heavier than
an O-360 installation. Roughly 925 KG or more for the 20B. The gross  weight
will go up more because of the required ballasting in the tail. The stall
speed will go up making it harder to land in short fields. The take off
speed will be higher so it will use more runway. The landing gear will
be more fragile as it will be subjected to higher loads due to the higherspeeds required, longer time going fast on the ground and the increase
in gross weight. Your fuel burn will be higher so the range will be less.

Span 28 feet. Wing area 135 square feet. Wing loading 20 pounds per square
foot. A good bush plane is a Cessna 180 with a 37 foot span

If you keep the same gross weight you will take 60 Kg out of the payloadand still cut the range.

I suggest you keep the 20B and buy a 13B for the airplane. If you p-port
the 13B you will get almost as much power. About 240 HP at 7500 RPM. Get
a 2.85 to one gear box and a longer prop (about 1.828 meters). Lengthen the
landing gear for the longer prop and strengthen it a bit.

I'll do a 20B motor mount design but I don't recommend using the 20B in this
airplane.
-- 
Paul Lamar ...No rotor no motor.

I'd agree that the engine is too much for the airframe, but Paul, is
925Kg right for the FWF weight? Thats 2035lb's.. were you meaning
420lb's [925/2.2]  or 191Kg's??

Jarrett

Metric is not my native language :) Thanks for catching that Jarrett.

Should have been 100 lb /2.2 or  46 Kg more weight FW.

Or 420/2.2 = 191 Kg total FW.