I think I would start by recirculating boiling water through the coils to soften and dissolve any syrup or dried beer that had been left in the pipes. I’d probably leave it recirculating for half an hour.
Then I’d fill with the pipes with very hot sodium percarbonate (or cleaner of your choice) and let it sit for half an hour. Rinse through with clean water and catch the outflow. If the outflow contained any particles or was discoloured I’d repeat this step until it drained clean and clear.
When happy it’s all clean, sanitise with star san and flush through with clean water one last time.
Then I’d fill with the pipes with very hot sodium percarbonate (or cleaner of your choice) and let it sit for half an hour. Rinse through with clean water and catch the outflow. If the outflow contained any particles or was discoloured I’d repeat this step until it drained clean and clear.
When happy it’s all clean, sanitise with star san and flush through with clean water one last time.