[Arran]

  Before assuming that the mica caps have a problem, and that it is not a dirty tube socket, dirty volume control, or a bad solder joint, rule the rest of those out first before breaking into the IF cans, hair thing wire and old plastic can ruin your day. The best way to find out the exact sizes needed is by removing the mica caps inside the cans, bridging a trimmer cap across each winding, and then peaking the IF through the trimmer, do not touch the slugs. Then you can measure the capacity of the trimmer at it's setting and order the closest size of fixed precision cap, they do not have to be mica at 455 KC precision ceramic will work fine.
Regards
Arran