Yep, what Fossil said...Seeger makes "pre-raked" trees, sells them as "chopper kits" for a very high price--like $1500 starting...these kits negatively affect the rake-and-trail formula, and, consequently, the handling (particularly on cornering). Good looks don't justify the price or the safety flaw.
In a word, don't do it.
Are you changing anything else, like adding longer forks? If you are, you can compensate for the changes by altering the frame a bit.
With the bike standing up straight, and with 4x4s or some other arrangement in place to hold the engine's weight (and after you've replaced the fork tubes), cut the downtubes about midway between the gussets and the backward bends. To add more rake, arrange a heavy friend or two on the bike at the seat, and while putting as much weight as possible into the seat, bend the backbone back and up (hint--remove the top motor mounts; another hint--a small cut in the frame might help, just don't forget to weld it up). Using turned-down slugs and tubing, fill the newly-created gap in the downtubes, starting with the top, tack weld, then remove the engine and permanently weld.
I imagine if you want rake but no length, simply make a slice cut in the backbone's bottom near the neck, and a corresponding "piece of pie" cut, about 1/4 inch at the widest point, on top, saving the piece. This time, put the weight on the top of the frame at the neck; when the top cut closes, you should have a rake change you can live with. Use the "pie" to fill the bottom gap, and weld it shut.
This is not a very exact procedure, but it works, and it's cheap, too...
