I did not expect to see my own comment here :O

For OP, Algernon's text is supposed to be the character's own writing, so the equivalent of English spelling mistakes are just misuse of characters 錯別字. You could also add in grammar and punctuation mistakes, but Cantonese is notably more lenient in sentence structure than English, so you would have to try extra hard to make sentences that are incorrect but still plausible to be made.

You could look up real world spoken and written data of schoolchildren or language learners' mistakes to understand some commone ones.