Before you do anything, you may want to -
1. replace the crank bearing seals on the engine;
2. lube the brake cables or replace as needed;
3. lube the throttle and speedo cables as well;
4. get your brakes in excellent working order with new shoes if needed and maybe a light sanding of the drum/hub;
5. are your tires in good shape - replace old tires;
assuming it is running well otherwise, your next step, if you choose, may be getting an exhaust. MLM people's pipe is a good choice along with a new exhaust gasket. The current crop of MLM pipes are made for a cylinder with a bigger exhaust opening. So to get it to work on the stock exhaust, take a dremel to the exhaust opening and widen that circle by a couple mm overall. Stop and test fit the pipe every couple minutes. Clean the cylinder well once you have finished. You may need to change the jetting in your carburetor with this new pipe. You will also want to change the drive belt and lower the weight in the variator to somewhere around 30-33 grams.
Since you'll need to remove the cylinder to do this, you will want to check condition of cylinder, piston and rings and, at the very least, replace the head and base gaskets. Scrape/sand/remove all of the old base gasket off the cylinder/case. Properly torque your head bolts to spec upon reassembly.
there's a start.(edited)