I'm not an expert but have had a few boats. Is the hull a welded construction or riveted panels? I don't run the rivers you mention but eventually you beat these things around and our old riveted hull had plenty of leaks as it aged. if you have an auto bilge pump this is not so much a problem but something to consider.
On a one man john boat the Yamaha 60 will sport along pretty well I would think but if you add people or a load of gear performance will suffer. Gear space is also important, we have an 18.5 foot ATEC Marine tunnel hull and when we pack a river camp on board the space fills up quick. Motor size computes into 2 things, time via speed and fuel economy. Our boat came with a 50 HP Honda which gave a 35 HP jet thrust. When fighting the currents on big swift rivers the boat used quite a bit of fuel due to the fact you were running at nearly full tilt to combat the current. It is an 80 mile trip to our cabin and the trip used 20 - 22 gallons to get there and about 14 to return. The time could run as high as 6.5 hours or better on the way in if the water is high and raging. We are hoping that with the new 90 HP motor we may see better fuel economy because the boat will jump up on step and then I expect to throttle down to a cruising speed of about 17 - 22 mph depending on how I work the water.
Learning these rivers and where to run for optimum speed and fuel economy is a big part of this game. You'll notice that fuel is a returning theme as I go on here. That's because fuel weighs a lot and the more you carry the slower you go and the more you burn until you burn down to light weight......... catch 22. Also Bush fuel is ultra expensive and the further you go the greater importance availability of fuel becomes. So, talk to some people who run various rigs and see if they seem to know what kind of mpg they are getting. Some guys will shrug off the question and fire off an off the cuff answer without having a real grasp of how far they travel on their gas. I run lots of long trips so I had to learn the ropes well.
I like the boat in the link, it looks like it'll get up and go but like I say try to find someone who is running one and get their input. One other thing, is that a tunnel hull? that matters if you gotta run shallow.