Antique Power magazine is dedicated to old tractors from steam models onwards. They have a huge classified section. If it was me, I would try to get a few copies and see what is for sale near you. Steiner has a large selection of parts for older tractors, so before you buy one, check to see what...
Since you have a Jubilee, I assume it is a 1953 model. If the transmission is similar to my 1952 8N, you cannot change gears on the go. You have to start off in the gear you want. My owner's manual is clear about that for the 8N.