First thing to do is ditch the base gasket and make one out of the thinnest gasket paper you can or brown paper grocery bag with some sealant on it.
The a cylinders respond really well to a little bit of porting, go easy on the intake but you can go quite a bit wider on exhaust.
Next is bigger carb and a pipe, an SHA will fit on the stock intake manifold, you might have to waller out the holes to clear the frame and replace the idle screw and spring with a set screw and jam nut. You can get a very cheap chamber on treatland to slip on the header pipe.
If you do that stuff right just about any Sachs will do 40, clutch will still suck so the take off won't be great but they run forever and make for a nice stock cruiser.