The 18" span models are on the small side for success though it has been done. The heavy draggy floats would make it considerably more difficult for that particular model.

As you say it's a lot of work and because the motor, battery, radio gear etc add a lot of weight you'd need to start by lightening everything. From the Guillows kits I've seen that might well mean throwing all the heavy wood away and getting some decent contest weight balsa to remake all the parts.

Probably not worth the effort.

BTW I think you'll find that's a .020 they recommend, e.g. the tiny Cox engine, not a .20 .

Martin, I hope you don't mean you're thinking of putting a Speed 400 in a 40-size Hangar 9 arf. It won't even taxi never mind take off.

Steve