Does anyone have any suggestions on how to reinstall the valve gear into the cylinders? I took the loco apart to give it a good clean as It was running erratically after being stored for 3 years, and the valve gear came out of the cylinders, and I am unable to reassemble it. Any suggestions will be appreciated, as I'm in the UK and I don't really want to send it to the US for repair as it will likely be very costly.

There is certainly a tech somewhere in the UK who can reassemble it for you. A Google search will probably turn something up.