I spent some time racing shifter karts in WKA and SKUSA, before I got seriously into motorcycle racing.
I'm thinking that a modern, lightweight 600 (or 636) cc inline-4 would be the engine of choice. Still well over 100hp, but lighter and much less torque stress on the chassis, than that old GSX-R 1100 mill in the Gixxer Kart. Torque doesn't seem to be something the kart would need that much of, seeing as there is already "on demand" wheelspin with just a wimpy little 39hp 125cc two stroke TM race motor in a typical shifter kart.
I'm thinking a crude but modern interpretation of a 1970's tube-framed Cosworth DFV powered F-1 car, but on a 50% scale, but way less than 50% of the weight. Properly set-up and driven, I'm thinking I could make a credible attempt at a sub 1:10 lap at Willow Springs.
Somebody please help me, before I start hitting the junk yards and end up hurting myself.