What year is the sled? If it's your avatar picture, it's not even worth trying to get more power out of it. You would never gain enough with it and parts for anything that old are just luck if you can find them.
1974 Arctic Cat Cheetah 340
Gonna be set up for ice fishing. Should be good for it's purpose!
Most snowmobile problems are caused by a loose nut connecting the handlebars to the seat.