Hiya Jet_Pack
Welcome to the site.
If you're building a trike from scratch you can fit anything you like, so personally I'd go with the complete K7 package, (way back in my teens, one of my best mates -"SpeedMaster" on the forum, had a K7. A huge bike -he's a big bloke, so it should certainly be up to the job. We tend to go for front ends from bikes of ...say, 650cc & over for our little Reliant trikes, just because they're designed for a decent sized bike so should be strong/stiff enough for our purposes. I'd look for fork tubes of 35mm diameter or bigger. We kick the front ends out at about 45 degrees, which means the forks aren't travelling up & down as much as they would on the standard bike. Skinny little forks could possibly flex in that situation so bigger is better.
A headstock is only a tube with a bearing at either end. Any competent engineering shop will be able to machine you the bearing cups so you can fit any bearings you like. There are several guys here on the forum with lathes who can produce bearing cups or steering stems to your design, (I know Twisted Patience is happy to do mail order jobs from his shed).
Most bikes use a taper roller bearing both top & bottom, (the bottom one's usually bigger), & someone like Dracken1 from the forum can probably supply you with a set to suit any front end you choose for 20 to 30 quid, (he runs JW Motorcycles -the link's on the Links page of the main site).
-hope that helps.
