Can old cars be remapped?

I have a honda civic ef 1991 for a while now just thinking will it be possible to remap it

Yes. If you replace this car & You should make some different with your good ideas.

Try to do it on eBay

"Remapping" requires a ECM that is programable. I doubt this car has such a ECM. A lot of "Tuners" put in an aftermarket ECM such as "Mega Squirt". Some cars have ready made "piggyback" units that can to some degree be used to "remap". I know there's a "BIPES" unit made for some Mazda products so there might be a similar unit for a more popular car like the one you mention.

Your car is OBD1. I don't think that generation of computers is smart enough to be remapped.

Not if they have a carburettor. Only cars with an ECU can be remapped and maybe not all of them.