Definitely can do the fabrication and definitely enjoy stuff like this.
The issue is I'd want to just get the $$ out of my dialed '02 that I want some components off of, so I'd have to buy a second RFS bike to make it work. I found some cheap ones locally that will do great but they're also older style '02ish.
I might consider sending the engine over to DJH to get checked over and possible a 540 kit and the stainless valves. Not sure if the '02 means much in terms of weaknesses. The only one I see if that the '02s don't have a Rekluse that can be used. Which I may or may not ever use, my frineds on the Rekluse love them and say I should get one but I've never really used one.
I have a 41mm carb already so I'd also get the head bored out I figure as well.
So I'm definitely definitely intrigued. I'd be about $4G into a 250 SX-F and a '02 520 EXC before selling anything...