Re: build suggestions for a logging road hill climber
Taking the needs into account, a 2 stroke engine with a reed valve seems the best way to go, offering better low end torque than a straight piston ported engine. Also, if you have no restriction on cc apart from the weight and size, something bigger than a 50 will be good as it will be less stressed under load.
It needs chain or toothed belt drive as well; friction from a roller or plain belt will disappear when they get wet.
I'd survey your options about availability of suitable engines and what engineering facilities and manual skills you have and then decide.