I have to second you on the
Winter pool cover Destroyer.

I use them as a Winter storage cover with a 2x4 frame running amidships, bow to stern.
My current project boat was under one for three years and held up. When I took it off, the boat was in the same condition as the day I covered it.
They outlast even the best of the poly tarps the chain stores sell. Besides being made of a heavier mil poly, (available in different thicknesses) they contain a UV shield that stops the Sun from breaking it down (the others end up looking like cheese cloth after a couple of months).
The oval shape
is the way to go and you can find all sizes on ebay (cheap!)
Even if you buy the canvas cover I recommend a
Winter pool cover to put over that for the off season. It will keep the Sun and other elements off the one you want pretty for boating season.
Speaking to trolling motors, I use a my boat for saltwater fishing and have a 1980 Johnson/Evinrude 15hp.w/power prop. The 9 -15 uses the same chassis (about 70lbs) so you can choose the power to weight ratio. I found a rebuilt
Electric model (ebay/$375). It has a 2amp. alternator that keeps up or even recharges the batteries in the event of trouble. She truly serves double duty- (trolling / auxiliary).
Heck, with fuel prices the way they are I'll be doing a lot of trolling this season.
Hope this helped...