Caster is usually the number one culprit.Another useful item (which may not work on Jenki's setup) is to replace the tension springs with compression springs.Maybe the internal friction or the higher spring rate somehow dampens the shimmy?
Yup,must get past the rings, as my drifter (inverted 503) with premix needs the plugs washed if left unused for more than two weeks.The oil injection on my 582 fox has an interesting quirk, the pump must leak but into one carb bowl.During inspections the rear float chamber has a red colour in it,and on startup will start well then run rough with a lot of smoke.My brothers 582 fox has exactly the same quirk.
Your cables sound very similar to kitfox ones.A push bike shop should be able to sell you some cable kits,the large ferrule may need some dressing down to fit.for the carb end use the sleeve off a stainless steel rivet,to clamp on ,use a brake hose flaring tool to clamp the rivet shank to the cable.This should be enough (I could hang my weight in them)but for safety I silver soldered the ends as well.If you make a pair on the bench when you fit them the adjusters won't be even(OCD I know!)as the bend radius makes a surprising difference. cheers paul
Many years ago I had a lubrication related failure in an air cooled non rotax engine.the shop that did the rebuild told me never use an oil specifically designed for liquid cooled engines in an air cooled engine. Oil technology has probably moved on,but just a word of caution. The 503 is generally a bullet proof engine(they don't make enough power to hurt themselves