Personally I would go with the rebuild. Have your cousin help you and do it yourself then it wont cost all that much really. Probably would need the crank ground and polished if the bearings spun, as long as its not too bad, otherwise it would need a new crank. Everything else is pretty simple and no real special tools or machining needed.

One big reason why I say rebuild instead of replace with used is you can get upgraded pistons with more and larger drain back holes for oil. That should help get rid of the largest part of the oil burning problem with these engines.