If you have the original objective lens there are some things you can do. Measure the outside diameter in mm. Measure the edge thickness and center thickness. I don’t know if that is an achromatic lens or not, so look at the edge and see if it is (laminated/ 2 piece) . Note if it is Biconvex or Plano (curved on both sides or just one and flat on the other. Then place a light source directly above a table or something, hold the lens horizontal with the objective side facing the light/ up, move the lens up and down toward the light and find its focal point on the table (make sure everything is in line vertical. Once you get a clear image on the table top measure from the center of the lens edge to the table. That is your focal length. Let me know this information and I’ll see if I cans find one (very likely).