Thanks for filling out your info... as Poboy mention this info is needed to help assist you, driving terrain affects what type of tire, tire size affects gear ratio, wt. affects gear ratio, etc...
Someone can help you figure out your final gear ratio (I'm not sure what pitch a 65 tooth sprocket is to suggest which size of clutch sprocket, plus factoring Wt.), but with your engine mods I would think if you stay under the 16˝ size tires you should be ok. A torque converter would be better than a clutch, especially with running on all terrain ground... which I also suggest knobby tires for.
I have a Manco 1 seater kart that has a 1˝ axle with a 3/4˝ step down. It originally came with a 7˝ rim with 16˝x8˝ knobby tires (16x8-7). These rims have the wheel bearings built in, and are held on with a single nut in the center.
http://www.gokartsupply.com/wlassy.htm
I converted my kart over to hubs (3/4 Step-down) and bolt on rims (4x4 bolt pattern), due to a type/size of tire I wanted to run.
http://www.gokartsupply.com/atvwl.htm
You could essentially do either conversion with your kart. The hub and rims are the more expensive route. You can see pics of my old rims and conversion in my build off thread...
http://www.diygokarts.com/vb/showthread.php?t=29364