Many mag probs are with the two lips at the front of the mag. Quite often they'll get pushed down from loading .
Try bending them up ( very slightly ! ) and check your load/feed until it loads /feeds properly. Also try and find some stripperclips ( properly known as "chargers" ).
Original parts ( buttstocks and a few other goodies ) are available at Numrich , who also uses a schematic diagram for parts , or Springfield Sporters , and IMHO , both reasonably priced.
Here's a good "how to" site
Collecting and Shooting the Enfield No. 1 Mk III - HISTORY
I'm quite happy with 1 1/2"-2" groups at 100yd in a supported shooting position.
Remember , your lowest sight setting is 200yd . At 50-100 yd , try aiming about 6" low and center , until you find your sweetspot.