Better off saving your money to buy a bigger motor, but...
The parts off the 30 kohler will fit. You can get the heads/intake/carb and you will see some better performance. Not gonna gain much speed, but should help out a lot with a load.
Other option is to pull your heads, take them to a machine shop and have them shaved .030-.045. I have a guy locally that will do this for about 100 bucks. Anywhere 100-175 or so would be normal. Have them check your valve seats also, and do a valve job while your in there if needed. Valve job is probably another 100 bucks. Stock valves and seats are usually okay on the kohlers but def have the machine shop check everything while they have it.
Another option on the carb is a mikuni. I have had a bunch of trouble trying to tune mine, but if you know what you are doing that is an option. The 30 carb is a lil more straight forward. Bolt it on and it's pretty much set and ready to go.
As far as exhaust, Mud Buddy is your only option. It's not a dual set up, but the single's flow better than duals anyway. Give Keith at setx performance a shout and he can hook you up. If you can't find anyone local, he may be able to help with the head work also.
www.Extremevtwins.com is another place that can help you out. Also Julian at
www.midwestsupercub.netGood luck.