rotavators

hi can some one help me with my howard gem 500 rotavator it has a jap engine single cylinder and a dry sump which drives by oil pump . my timing is out as it keeps back firing points gap is ok but I have taken the timing chain of to clean it  I think this is the problem can some one tell me how to set the timing back up .
or I will have to take to the repair shop.
thank you all rotavator

hi thank you for that I hope it will help me to get it running again as all my does is back fire when I try to start it
thank you rotavator