Off the top of my head I don't know just looked on a couple of websites and I tend to think it's just one repeating another's mistakes.
I do know someone who will know I'd try a call to Boom Trikes at Boothstown. [You must be registered and logged in to see this link.]
The sell and used to make and sell their own design trikes plus I know they have one if not two Dandys.
A lot of the stuff on websites I think is confusing trikes and bikes with side cars.
As for the engine no problem.
If you get somewhere with Boom please re post and tell us what the outcome was if not come back and I'll look further.
Sorry I was doing something else when I first responded so not my full comprehension. If you already had the Dart and Designer and a previous trike did you tow them with this or any trike?
If so you hopefully know more about the legislation on weights than us. In fact while I run a few bikes trike owners are really thin on the ground so why might we know more?
A 2 litre engine automatic or not would have no problems in nearly all cars towing a Dandy. The only limit being cars that were underpowered even without towing. The power to weight ratio on a trike must be far better.
Nice to hear from you though. As always be it trike or Dandy. We love photos! Though you can't post for the 1st week. If you have some cracking photos say so and I'll send you an email address and I will host the photos for you.
Have you used the Dandy since your back problem? I've got a duff back and some stomach muscle problems. Lifting the front and rear of my Designer can strain a bit. If you have any issues come back to us there are possibly ideas to pursue.