fault you have is common, it is the 3rd /4th gear shift plates you will require 3 shift plates and a 4th gear synchro ring, it is caused by heavy and abusive gear changing.if it is just forth it can be done without stripping the box,now they are old 3rd gear synchro is often damaged by the broken shit plates,to replace this you will need to strip the box getting 5th gear off can often destroy it and they don't make them any more .I would advise due to their age that they are FULLY rebuilt you cannot Fully rebuild a box for 250 including labour it is obvious that hardly any parts were changed ,so a repaired box then