John, welcome back! The man you need to speak with is Bruce Longman, contact derails on the inside front page of Cheval. Be aware he’s been a bit ill recently, so may need some time to reply. Not sure if he has rods in stock at the moment. The Rods don’t come with shells, you have to tell him the size of shells you want to fit, and your engineering shop will then grind the big end journals to that size. As you say, you won’t know until it’s all apart.
Square 4 mk1, Square 4 mk2, Ariel Colt, Matchless G12, BSA C12.