Replacement parts can be found. A #4 sight will fit on, but will not have the right settings due to the shorter barrel. The No.5 sights only go to 800 yds.
The going price for a No. 5 in near new is about $500. Obviously, yours is worth much less. Try
www.gunandknife.com for their Enfield forum. They have lots of info on parts and prices.
Jim