to replace the muntins, you will first need to find stock that matches the existing....if you can't find the stock, you may or may not feel comfortable getting something that matches closely....if that doesn't work, you can make a reasonable replica with a table saw, plane and chisel....you will then need a coping saw to cope the end of the muntin to fit snugly to the adjoining profile...
you will need to remove the sash from the opening...typically an aluminum jamb is nailed, screwed or stapled into the jamb at the top, middle and bottom of the length...once removed, it is typically impossible to reuse....blaine carries a number of jambliners:
http://www.blainewindow.com/BWH0021.php ... D_filt=A62it is possible that there are pulleys behind the jambliners and a space behind the jambs...if that is the case, a rope, weight and pulley balance system is your best option...
good luck.......
.......jade