Your choice of parts depend upon what you intend to use the bike for.
I would say, for looks go for the Triumph pre unit, if you want to ride the thing any distance go for a unit motor. I can't comment on the Norton motor as I know nothing about them.
Regarding forks, jap parts on a Triton would look out of place, go for the Norton roadholder forks if you can afford them, if not, early OIF Triumph forks are cheap and look the part and they can be modified to take external springs if that's the look you're after.
Bear in mind that the frame and the motor are the cheap parts, it's the rest that cost's money, a rear hub (Triumph conical for example) starts at around 100 bucks, or you could go for a replica Manx hub at 2,000 bucks, the sky (and your wallet) is the limit. It's the same with front brakes, you can still pick up a Sukuki GT750 4LS front drum for around 300 - 400 bucks, personally I like the Gremeca 260mm 4LS that can be bought new for about 600, or again you could try and find a Yamaha TZ 280mm drum for around 2K or a new Fontana for about the same price.
If you need any further info and /or link and contacts let me know.
Webby
(71 Bonnie and Wideline Triton Unit 650 in progress)