I say go for a GEBE kit.
Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More
Light weight, reliable for 1000's of miles, virtually no maintenance, no chain to rust, clean and lube. If you choose a 2 stroke engine it is both light weight and does not care if you carry it on it's side or up-side-down while fording a river or crossing a gorge.
Certainly not the fastest around but plenty fast enough and you said nothing about wanting speed.