if memory serves me correct, the 2 strokes run on 16.1 mix ...but check before you run it on that , just in case I am wrong !!! They did do 4 strokes as well , so make sure it is a 2 stroke . Here is a link that should be handy for you :
http://www.villiersparts.co.uk/allen.htmlLater models were fitted with Kohler engines , never seen one of those fitted though .
I hope you don't suffer from arthritis !!!! Allen Scythes are real bone shakers , to say the least . Fantastic old things though ...good luck with it .
cheers
Russ