I raced Rotax Max for 5 years. The parilla might be the way to go. The cost of the rotax parts are JACKED upby the importer.
Rotax's will run strong for 50 hours before a bottom end,
If I had my choice for a high hp fun kart it would be a Bilandsa250
YouTube - Nick@Tibury 10.4.10. Biland
It makes 26 hp wich is down 2hp from the rotax and leopard. and only revs to I think 10,500 where the rotax will run to 14,500 and the leopard a bit more.
Where the Biland rules is torque, It's like having a bigblock Chevy in a Mclaren M8
I rented one for ten laps at a track. It would step out a litlle coming out of the corner! My Rotax would bog out of the corner.
They also run 200 hrs between majors.