You can see the vent nipples in that pic, if there is any white spew there, those caps are done for. If ther eis no white spew, you can probably sell the caps for more than new better working replacements. Because of the way Hiwatts are made, replacing is a DIY job as long as you can solder and know the difference between neg and pos. Suitable caps and about £3-4 each for the big ones, and more for the dual one, I'll be ordering stock of those soon.
Edit: Also, I have spare transformer sets suitable for the Sound City or Hiwatt, be about £500, so you know the odds you are playing.