right i have 2 gearboxes,,,,, bother not great

ty52 box that whines and spits oil when driven hard AND a ty54 box that has just spat out 3rd gear within 6 weeks

now im thinking of options here

can i change the 3rd from ty52 into the later box OR can i change the gears totally to the later casing ( guess the gears on the sti3 are stronger,,,,,, as the box just whined and lost oil)

or is there another 3rd gear i can buy to also avoid this