@Clavinavi is correct that a lighter bike would be better for off road, but people have been making scramblers out of Sportsters and other bikes for decades. I've been looking at making a scrambler out of a Street or Sportster for the last couple of years just to tackle some of the less improved country roads here in Iowa. I've seen some custom built bikes on other forums.
Here are some of my thoughts on building such a bike with mostly bolt on components:
In the thread Rear Tire 170/80-15 member @Cliff showed off the Pirelli Scorpion Rally STR tires (110/70/17 front & 160/60/15 rear) on his Street 750. These aren't true nobbies, but they are a fairly chunky ADV tire. Tubed tires are considered better for off road use, but I'm not really ever going off road.
Spoked wheels tend to have more give when they hit rocks and logs than cast ones which might crack. That makes them preferred for off road use. But laced wheels would run about $2K, which is probably more than all of the other proposed mods combined.
The Tuffside Brat/Scrambler Seat Kit has always been central to my planned build. It includes a tail tidy but does require some frame cutting and welding. I'd probably contract that out.
Progressive Suspension makes some slightly taller shocks which should take care of the back end. I haven't found a good solution for the front suspension.
Biltwell and Burly both make 7/8" MX style handlebars and pegs.
I haven't found many options for a high exhaust other than the Custom garage Full System Exhaust H-D Street Models from DDH Studio, which I kind of like. I think it uses the passenger peg mounting point, so you will be riding solo. They are straight pipes, so it will be loud.
A chain conversion would be nice to have, but there is no off the shelf solution. Since I'm not planning on ever truly off-roading it but just taking the road less traveled I'm OK with the belt.
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