Some years back, about 2005, Mert Lawwill built a very limited edition street tracker. It was based on a Harley Sportster, but much of the original bike was tossed. Since only twenty were ever made they don't show up for sale very often, but one's available now.
The frame was completely rebuilt with 4130 Chrome Molly tubing, only the portion with the VIN was saved, along with the engine mounts. The swingarm was replaced with Mert's own design, a dual swingarm and single shock that transfers weight completely different than a standard setup, forcing the front end down on acceleration, keeping the bike planted and enabling lots of throttle exiting the turns and on the start, wheelies are almost impossible. The forks are from a Buell.
The engine is now 1400cc and the heads are completely new, cast with the intake and exhaust ports on the opposite side from stock so the carbs and pipes are properly located to replicate the real XR750. The MegaCycle cams are custom made since the ports are reversed.
The "number plate" is actually the oil tank which had to be relocated due to shock placement. Excel rims with spokes and Brembo brakes on Maxxis tires complete the look.
These Lawwill trackers are about as sweet as they come. Not cheap, but the asking price is $37,000, which is what they were new and with 5400 miles, this one is pretty close to being new. It's a nice opportunity for someone. If I had the spare change, I'd want this one myself.
Link: Mert Lawwill street tracker on eBay