Personally I'd be inclined to ditch the bait altogether. It doesn't sound as though you've got an especially food-oriented dog and in any case, it is better that the dog understands what you want from it and responds to the command (whether it be a sit, a down, or a stand/stack) rather than trying to manipulate the dog into compliance with a bait. In that case, the bait becomes a crutch, rather than a means of getting the dog to look animated - which is actually the sole point of bait in the show ring
If Mugsy is trying to sit when you're trying to get him to stand/stack then that suggests to me that he doesn't actually understand what it is you're asking him TO do. Quite possibly he doesn't fully understand the 'sit' command either - or at least he isn't differentiating between that and any other instruction you're giving him.
I think you need to go back to working on teaching him different commands, working through a range in varying orders so that he has to concentrate on what it is that you're asking. If he sits when you tell him 'down' or 'stack' then he needs a negative marker. When he gets it right, then praise and/or treat - in fact, Mugsy could be a very good candidate for clicker training as that will let him know instantly when he's got it right.
Once he's learned to differentiate between commands and is reliable on that you can start baiting him, but I think right now it's a case of cart before the horse. As for what to use - personally I think cheese is good, or at least something you're happy to put in your own mouth before giving to him. That's a very useful technique for showing. Baiting is about getting the dog to look at you and show animation. That's most easy accomplished when the dog focuses on your face waiting for something yummy to pop out of your mouth and into his