Royal Enfield piston rings

Hi everyone, I have recently purchased an old mower it was sold to me as a ransome mower but after a little bit of research I have found that it is an Enfield mower, it is actually in very good condition, I stupidly however snapped a piston ring and now need to find a replacement, I’m in Geelong, Victoria Australia. Can someone point me in the right direction, I’m assuming a standard ring won’t work and will need an oversize one. 

Thank you in advance.