First, the best thing for you would be to remove paragraph "4". Nobody is going to help you when you come off as combative right off the bat.
To answer your questions:
Legends are air cooled, so there is no radiator. I know of people that have turbos and/or nitrous on the air cooled 1200's (along with some wicked VW beetles). If you put a liquid cooled motor in, only you will know if there is room.
I have found Legends chassis to be rather sturdy, and I have 4 years of some hard wrecks with the same chassis to prove it.
Yes, a full containment seat is possible, many people run them.
Theres not much to the cars, so not sure on what you mean by "upgrades" available.
Violent crashes...been there, done that. From 80-0 in 10 feet, and also flipping and snaprolling down the backstretch, hitting and breaking a retaining fence post, coming to a complete stop 15 feet off the ground, and dropping straight down onto my roof. Always walked away. Have video with people saying "oh my god, he's dead". I'm still using the same cage, it's well over ten years old and has never let me down.
I don't know the first thing about dwarf cars, so I can't compare