I had this bike(and hair) about 25 years ago, it was an alloy xr750 from the last batch made in 1980. I put dummy lights on it and a disk brake on the front and rode it on the street for a year. I also took it down a 1/4 mile drag strip a few different weekends. It had very quick steering, at highway speeds you were almost better off riding it with your hands off the bars because the wind buffeting on your arms and shoulders would make it wobble if you sat upright.
The frames on these bikes were lightweight chromemoly. I had an extra frame and swingarm and I think bare it weighed about 24 pounds.
Back when I had this bike the blueprints for these frames were floating around, a guy I used to talk to had got the blueprints for the road-race version of the XR frame and had made one up from them on his own. There was a guy in Chagrin Falls, Ohio named John Steel who was buying all these bikes up back in the 1980s and I am sure he is still into them, if you got hold of him he may be able to tell you how to get those blueprints as he was having replica XR frames made and trying to build and sell accurate replicas of the bikes.
Yea, this was a quick bike, with around 100 rear-wheel horsepower and weighing in at a bit over 300 pounds. Off the line it was about as fast as a stock Sportster, but when it came on the cam the power felt like it at least doubled and the front wheel would come right off the ground. No kick or electric start, you just ran down the street, jumped on and popped the clutch. The racing exhaust was loud enough that a couple of State Police going the opposite way down a highway turned around and gave me a nice ticket one day, not a bad penalty for a whole season of riding fun.
The iron motors will have about half the power of the alloy ones and they will be a LOT heavier. Back in the day no one could give away an iron XR, I knew where there was more than one of them that was brand-new, sitting because no one wanted to race or have them as collectors items. I suppose that changed once all the alloy jobs were snapped up. I had a few other old Harley XLR dirt-track racers too, I got rid of them all mainly because the parts are so rare for the engines that if you blew one up you would never find the parts to fix it. The looked like street engines but every part in them was non-interchangeable.
If I was going to live to be 150 I might put something together along the lines of the famous Aluminum Steamroller(google it) but using an early 90s alloy Sportster engine, I think that Aluminum Steamroller was the best cross between an XR and a street-sportster ever built. If you build one you better make it so it has almost too much power for the chassis and give it very borderline and quick steering so you can have some of the feel of a real XR like I had, don't build a no-go show-boat.