| So, which is it? BSA, Norton or Trumpet? With the frame at the angle you want the lower loop to sit measure the distance from the bottom on the fork stem to the axle center line of the size wheel you will use. You can use a long dowel or two so you can go through the stem to get the angle somewhat close.
Hope you didn't give a lot for that frame because any of those engines can be hard to find in good shape and are not cheap to rebuild. | 
