First of all, it IS a KLR!
Even the most ambitious KLR650 hop-up won't approach the out-of-the-box power of, say Husqvarna/BMW/etc. 650 singles.
Not sure what expectations may exist, but . . . increasing displacement, IMHO, offers the best hop-up bang for the buck. A reliable path toward obtaining more power remains: Processing more combustible mixture per unit time. Sad, but true, fact-of-life.
Another approach, seldom discussed in KLRdom, is: Improved combustion efficiency through raising compression ratio. Then, there's the old stand-by, reduce intake and exhaust restriction. Supercharging?
Altered valve timing? A trick cam can shift the horsepower and torque curves about the rpm axis, but a cam alone offers little, if any, net gain.
Regardless, approaching the peak horsepower of a stock Rotax 650 cc engine appears to me a challenge.
If you just want to multiply torque, change your gearing.
Best wishes, whatever hop-up path you chose.