1. The area where the fin was attached should be repaired like new before trying to glass on a new fin. Then surf it as a twin-fin for a week or so while you decide how to repair it - by yourself or using a competent repairman.
2. You could put an FCS or Fusion box in (preferably using an FCS kit - 135.00). It takes 3 different
bits to do a really good job - drill, grit-holer, and H-Pattern, and a couple other things. Don't knock this approach since the kit is really easy to use. You just have to buy or borrow one. The bits are really high quality and then you could surf your board as a twin or as a thruster. But you would need to buy only one FCS center fin which might be troublesome, unless you want to break up an old set of yours.
3. If you really want to replace the fin as a glass-on, you need to cut and/or grind ALL or nearly all of the old glass off to get down to the fin and start over. It's tedious. If you don't do this, then when you put fabric over the fin to glass it back on, it will be too thick - this would be a little bit ok if you want a thicker fin but it's probably not what anyone would want. You can also shape yourself a new fin and skip the grinding of the old fin until you have time to do it. If you're good at grinding, just grind it all off and get down to the original fin. You then have to cut about 3 or 4 sheets of glass (larger than the fin) on each side of the fin. There's another way that involves folding a sheet of glass over which is kind of cool - check Youtube. And sometimes the glass used is a different weave for strength. And you should use fin rope at the base. Repairing the area where it ripped off is the hardest part though. And possibly grinding down the fin or ordering a new one from a manufacturer. You have a lot of flexibility though since it's the center fin, and if it's not a perfect match, your board will work ok.
4. You can surf it as a twin-fin once the area at the base is repaired since it's luckily the center fin. You will have a shorter turning radius, and it will be squirrellier and you have a tiny bit less thrust from the missing fin. And for some people who have weak projection into a turn, you will have a harder time sometimes getting it to turn. But when it does turn, it will turn sharply due to the shorter radius.
5. Making a temporary repair is really tough without some sort of great epoxy. Someone mentioned gorilla glue - I don't know if that's strong enough, but you might lose your fin in the ocean which would be a shame.
It might work for a while if you do a really great job of the repair. But without sheets of glass on the sides, I can't see how it would last more than a short time.
A professional could probably repair it for a reasonable amount.