At 210# you are looking at rolling across the scales at 380-390# with a typical kart engine so you might think about a stroked sudam or even a stroke sudam with a 57mm piston. You would probably need to go with a Wiseco Honda piston and I would set it up run it to see how it does and then have it Nikasil.jmo
The Swiss Auto is a very excellent little motor. Fuel injected, electric start, water cooled very low maintenance. I've got 10 hours on it and never adjusted a single screw. Start up and race.
BUT,, I weigh 225, and weighed in at 440lbs which was about 70lbs over the min. Great on the short track for me, and I'm sure it would make a nice all around motor for someone that could get closer to the weight min.
Big price tag, but without it, I wouldn't of finished any races this year.