You sound like you have a bit of challenge on your hands... you dont say but I assume you will be taking the bike off-road?
toothed belt would indeed seem the best way to go as it would be the quietest (no chain noise) and a fairly acceptable range of engines..
considering that you are running in those temperatures you might need to increase the % of oil slightly - in my case I store the bike in the flat so its always around 20c ambient temperature - but the engines are usually pretty good for starting..
With the fact that you need a lot of torque that could present problems with a belt drive as you have a more limited number of ratio options - but GEBE do a set of gears for their engines from highway - trail gears. The latter would probably more suitable...
Its possible that you could get an mount only kit from Golden Eagle (GEBE) and fit your own engine, but what you would be looking for is a engine with alot of power and torque produced at lower revs... a 14000rpm engine producing 4hp will be alot less use than a larger capacity 4hp running at 8000rpm
I don't intend to ride off road in the winter. Larger snow falls where the snow is not packed is the main issue. Side road intersections can bring my bike to a grinding halt in quick order. Throw in the wind when it's cold can make things really interesting.
Thanks for the oil tip!
I have heard of the Gebe boxes, I don't really know too much about them. I assume they will work with either a frame mount engine or rear rack mount. As for engines I'm thinking something like a GX35 or GX50, I hear that they are a similar size.