Yes, I would agree it wouldn't be my first choice for a Kart or mini-bike, but I'm confident that if that is all I had to work with - I could make it work.
Not interested in getting out the pointy crayons to do all the math just to prove a point on a forum, but I do know this - if it takes "X" amount of apply force to a given clutch friction area to transmit a given load to a clutch drum - how you get that apply force can be manipulated by the weight of the clutch friction "shoes" - the centrifugal force of heavier shoes at a lower RPM can match the apply pressure of lighter ones at a higher RPM. To over-simplify - shoes 2x as heavy will apply the same force to the friction area at 1/2 the RPM. So, I don't see applying the clutch to lock up at the lower RPM as an issue. Actually, locking up the clutch at 1/2 the RPM is the easy part - the REAL issue is that - given the 2.0:1 gear reduction - it now has twice the torque to deal with. You would also have to change your final drive ratio from 6.0:1 to 3.0:1 to get the same speed - further loading the clutch harder. I guess running 2 clutches properly sized and built to lock up at the low RPM would fix that, but realistically how far do you go to fix a problem you created by using the "wrong" engine?
Since I've went this far - IF I had went this far and HAD to put the "wrong" engine on a project kart, I probably would not re-invent an RPM based drive system. I said I (see "SMART guy reference LOL) could, not would. Personally, I would use a belt driven jackshaft "clutch" setup, with a manually controlled spring-loaded idler pulley, like a rider mower. Think WOT clutch drops to get the party started.
The FINAL thoughts are - the O/P asked for thoughts on this engine and I gave them.
It would not be my first choice.
I COULD make an RPM based clutch work if necessary, with some creative engineering and a government budget.
I would probably choose an "analog" belt driven clutch setup for engineering simplicity and cost.
I, being a fat "kid", like cake . . .
Enough time spent overthinking a project that won't (shouldn't?) be built.