My experience with Cushmans is limited to examining machines that belonged to other guys at shows. My impression is that they are simple, and solidly built, but I'm definitely no expert. The two speed transmission is said to have reliability issues. I believe that the two speed was the top of the line and the other option was a centrifugal clutch. One Cushman I saw had a Briggs Vanguard V-Twin installed, and the owner said that this machine was capable of freeway speeds and that his uncle who had built it, would ride it cross country to Cushman rallies. Another option, if power is limited, is a Continuously Variable Transmission, such as is used in go carts. It's also possible to install a gearbox from another machine, which is something I'd consider if I was doing a resto-mod rebuild. Motorcycle engines can be reworked into standalone transmissions, by removing some parts and exposing the input shaft.
If the engine has been replaced, presumably the drive train has all been replaced as well, and reliability of engine and trans. if well executed, would be that of the replacement unit. Although this lowers the collector value, it's probably the wiser path for a vehicle that will be used hard.
I would put more faith in the long term survival of the Cushman, over the Chinese machine, but in either case, I think one is kind of on their own in terms of mechanical support. No dealer and not many mechanics would be willing to work on a vehicle like that. Practically speaking, for use in a small hauling business of
electric tricycle for adults, I think one would be better off with something like a Dodge Dakota or Ford Ranger. You'd consume more fuel, and it wouldn't be as much fun, but the vehicle would be an expendable tool, rather than having your blood and sweat in it.
The interesting thing about the electric trike, to me, is that it might be over the line of what would be considered to NOT require registration, or insurance, or a license to operate. For a those of us who might be approaching the age where we may be required to give up our driving licenses, it's nice to still have some options to do what we enjoy.
Here's another idea that I've been thinking about: