Check beather pipe first (located on crank case top) if its hard,have crack,loose etc oil will sludge up
For flush
Get some 20w-50 oil mix kerosine oil 70:30 ratio (700ml oil 300 ml kerosine)
Put it in engine start engine and let it idel for 15 min don't revv engine
Drain oil and put 1 liter fresh oil and use it for 100km max drain that oil and clean centrifugal oil filter (located near gear level)
Your engine will be cleans as new
BTW avoid short trip