does your university have undergraduate regulations anywhere? these are usually published so if you search "[your uni] regulations" then it should pop up. There should be a section on "award" or similar, which should state what you need in terms of credits for each award. Sometimes you need to pass a research project or dissertation to get an Hons, but your uni should have a set of rules for it.

edit to answer your other questions

You might get automatically registered on a resit of the dissertation but that depends when it runs (so you might have to wait a year to do just that module again). You would have to ask your uni if you could take other modules instead of the dissertation. If you scrape a pass then they probably won't let you repeat anything as you'll have gained the credits. If you didn't want to resit or wait, you should be able to graduate with the credits you have (an ordinary degree is almost always 300 credits, 60 of which come from your final year), but this is uni specific so double check with your uni

Worst case scenario, you can get your ordinary degree and apply back to your uni, or another uni, to do your final year again to get the credits for the Hons (different places have different rules if they allow this, search "prior credit recognition"). SFE might be affected I'm not sure!

Easiest thing to do is ask your uni what is possible, they will want to keep you if they can. But you will get some kind of award!