As you examine Lydia the cockroach you realise what you have done is wrong and you have to at least try and put it right.
"I'm really sorry Lydia, I honestly didn't mean to turn you into a cockroach. I meant to turn you into a...well, it doesn't matter. I'm going to get you back to your old self as soon as I can."
Lydia does not respond in any meaningful way and you know there is no way of communicating with her now. You have no idea if she can understand you or if she now has the intelligence of a cockroach. Either way you don't want her trying to escape again. You see a tip jar on the table and replace the loose change that was in there with Lydia, who scurries about at the bottom of the jar happily enough. You assume. You pick up a clump of grass from outside and put it in there with her and looking through the odds and ends lying about the tent find some film to place over the jar, remembering from your days as a bug collecting kid to make breathing holes first.
With Lydia secured, you now have to work out how to reverse the spell she is under. Kissing a cockroach doesn't seem like a practical solution, but you figure you didn't have magic powers when you woke up this morning so someone or something at the Fair must be behind this. You just have to work out who it is and how to persuade them to restore Lydia without transforming you or anyone else.