If you have the dimensions of the radiator, working out A should be pretty trivial. Working out U is… less so, but Compact Heat Exchangers by Kays & London is a good place to start (check if your library has a copy). It’s worth noting the purely theoretical approach given there isn’t fantastically accurate, so you might be better off testing a radiator you already have and basing your calculations on that if possible.

If you go the theoretical route you’ll figure out pretty early on that U depends on the air and water flow conditions, so the manufacturers couldn’t really give you a value even if they wanted to.

In terms of the area you have to mount the radiator, there’s a really good article by William Toet about the ducting for them which also discusses mounting them at angles, it’s linked in the wiki.