There are liquid cooled NC300 4 valve OHC engines for sale that make 30+ horsepower, which is plenty of power to make 90mph or more. Heck, go for a 450cc engine! The problem is that they put these engines in more expensive bikes. These engine have 6 speed transmissions with gearing appropriate for a dirt bike, dual purpose, or road bike, and LOTS of useful torque.
This would be a great starting point for such a project. There would be plenty of need for fabrication and frame modification if you get a used dual sport with a bad engine. There are lots of those out there, but hard to find because they are not worth much and just sitting in the garage or under a tarp. You would need a radiator or two. The benefit of motorcycle engine design is thin walled aluminum case halves (significant weight considerations), internal stator/rotor, ignition trigger, Internal clutch, integrated gearbox. ALL of these things help keep weight way down, but also allow room for a carburetor in the frame space.
We have some threads about adapting (motorcycle) engines to frames on this forum. These projects can take a year or more! I guess my point is that you are proposing to build an albatross that won't really be good as a dual purpose bike. It could be better as a road bike, and maybe that is OK. Frankly, I am having trouble envisioning the process of taking a cylinder from an i6 block. That engine uses accessory belt(s) for alternator (etc), don't even have a stator with a CDI setup, including an ignition trigger. You are truly reinventing an entire engine before you even get to the point of adapting it to the frame. It will be nothing like a KLR. Are you thinking of using an accessory belt for generating voltage to power the charging system and ignition? And then there is the intake and exhaust issues. You would have to somehow engineer and fabricate the intake manifold (single carb) plate on the head, as well as a single exhaust port both with mounting studs. You would have to fabricate the exhaust as well.
Also, those i6 blocks have much thicker crankcase walls (iron! = weight) and cooling ducts. You will have to have a radiator if you can seal up the block. And where is the water pump going to be? I guess you can use the end of the engine that does have the external pulley and set up an accessory belt with water pump. This is going to be heavy and large! Gee! And as I said, then you have to get it into the frame! Aside from the cooling, ignition, charging system, there will be a gearbox bolted to this engine, and a clutch between them! Whoa. Maybe I am missing something here, but integrating that cylinder and crank with all of this seems impractical, if not nearly impossible.
I guess if Allen Milliard can adapt two cylinders from a Pratt and Whitney radial airplane engine into a 5L V-twin, I guess anything is possible! It doesn't look like a dual purpose bike though lol!
https://www.youtube.com/watch?v=I-Xr1bmbZ_s