Hi Wess123, I suspect that we need a little more information but the most likely cause of the problem you describe is that the decompressor lever is out of adjustment. (please see the attached Lister instruction manual page) It could be that the valves are out of adjustment and fouling on the top of the piston at top dead center.
You claim that it has never been overheated, difficult to do with an air cooled engine, unless what you have is an STW1 which is water cooled. Water cooled engines are easy to overheat, inadequate fluid/water pump/fan belts etc.
You do not specify what your engine is coupled to but the fault could be with the driven not the driver.
Other possibilities include fluid in the cylinder water/oil/diesel. Excessive carbon build up on top of the piston, this is common in engines that are only lightly loaded. dribbling a little water into the air intake when it is running and hot will usually clear all but the most suborn deposits.
Let us know what you discover.
Bob