http://www.motostrano.com/Supermoto-s/2852.htm is a great source for parts and complete SM packages. If you are going to ride it on the street you might want a stronger lighting coil setup .
http://www.electrosport.com/ is a great source for lighting coil and rectifiers. AJ has a relationship with them so check with him first. Also on the street cooling can be an issue if you have to sit in traffic. I believe the 450X has a fan, but an overflow tank would be useful, and you might want to talk to AJ and find out if the Boyesen water pump upgrades are worth the money. Adding one of the extra capacity oil tanks that the desert guys use would be useful as well on the street. Extra oil capacity will help keep the temps down and will also give you a little insurance as these things do use some oil.
17" wheels are super awesome, but you'd be shocked how good these bikes will work with a good street 21" and 18" tire on the stock rims. Motostrano will be able to hook you up with either.
In my mind the most important thing you can do is buy the biggest best brakes you can afford. You can never have too much brake or too much lever feel (a function of master cylinder piston size and caliper volume/sizing) . I use the stock Honda master cylinder with a Motomaster 320mm floating disk and the Motomaster caliper.
http://www.motostrano.com/Motomaster-Supermoto-Brake-Caliper-Adaptor-p/m104.htm
I always thought it worked really well but after riding the Ducati with the Monobloc Brembos I got an idea what super powerful brakes with tons of feel are like and I realize the money for a master cylinder is totally worth it. An upgrade is in my near term future.
Gearing is the other thing that is tricky. I'm not sure how the internal ratios differ between the F & X so I'd check the SM forums to see what guys are running on the X .
AJ has built a few street legal SMs so I'd talk to him as well . I'm sure he has insights I'm missing.
The forums @
http://www.supermotojunkie.com/ were a good source of info in the past. I haven't checked in for a while but it's worth a look.
There used to be some SM guys on Thumpertalk but I haven't been there in 5 years or more so ....
If I can help in any way just let me know.