Wouldn't an electric front hub motor serve him well ? I'm thinking for it's reliability, ease of op's, simplicity, weight, tooling required for maintenance, and ease of recharging for him. These considerations would fit into the parameters
of his needs, limitations and safety and provide him with a successful first experience with a motorized bike.

If he wanted to go on with a fuel motor then he could pursue that and understand from reading this forum and others like it what he's getting into
with these Chinese motor systems knowing he will likely be his own builder/mechanic. (I'm sure others would be willing to assist him)

I just don't know if these geared electric hub motors provide for a dynamic brake
when throttled back. I would think that could be a pluss for this fellow too.