I LOVE the passports. they can be tricky registration-wise depending on state.
as mentioned there's a lot of exra to learn working on them, but it's not particularly that much more difficult. Timing chain to remove the head, valve adjustments etc, and all the gears if you rebuild, it's a good in-between for learning mopeds. parts are super easily available but not as cheap as one becomes accustomed to with mopeds.
lifan swap to me is a pretty huge copout but they're fine engines. it's the lazy-man's kit for these. Most of the people, at least younger folks, riding these are coming on this wave of enthusiasm for them, they're a cool "and" bike, like they fit as an accessory if you already have a big bike - they're cheap enough and small enough you can have both. And right now theyre very trendy to build customs with in the moto-dude-blogger sphere. that said, most of those focus heavily on frame mods and big custom aesthetics, while nobody wants to fuck around tinkering with the old 45mph engine when they can drop a grand on a complete turnkey 150cc pitster pushing 15hp. From a motorcycle perspective a $600 performance engine is nothin' and it's more a showpiece than a regular commuter so how it works matters a lot less than how it reads in a write-up or showing off to pals.